“For of all sad words of tongue or pen, The saddest are these: ‘It might have been.'” This is a quote from 19th century poet John Greenleaf Whittier, and it rings very true in just about every aspect of life. That includes the NBA.

There have been many players throughout NBA history who were simply unable to reach their potential, whether it was because of injuries, off-the-court issues or whatever.

Here are the 10 biggest “what if” players this game has ever seen.

10. Ralph Sampson

Ralph Sampson had it all early on: College legend, No. 1 overall pick, a four-time All-Star right off the bat. However, thanks to knee and back issues, it didn't take long for Sampson's NBA career to take a turn for the worse.

Along with Hakeem Olajuwon, Sampson helped comprise one of the most dominant frontcourts the league has ever seen with the Houston Rockets. In 1986, the duo led the Rockets all the way to the NBA Finals, where they lost to the Boston Celtics.

Ralph Sampson
Photo by Brian Drake/NBAE via Getty Images

Sampson was a 20 and 10 guy in each of his first two seasons, playing in all 82 games both times. But in his fourth season, the University of Virginia product was limited to just 43 games because of injuries.

Houston would proceed to trade him to the Golden State Warriors midway through the 1987-88 campaign, and he was never the same afterward.

Sampson went on to play nine NBA seasons in total, averaging 15.4 points and 8.8 rebounds per game.

9. Brandon Roy

You won't find many silkier scorers than prime Brandon Roy. The problem is, his prime didn't last very long.

Brandon Roy, Blazers

Roy entered the league as a first-round pick of the Minnesota Timberwolves in 2006, but he was immediately traded to the Portland Trail Blazers.

He asserted himself as a dynamite scorer very early, making the All-Star team as soon as his second season. By his third year, he was averaging 22.6 points per game and logging .223 Win Shares per 48 minutes.

But during his fourth year in the NBA, knee injuries began to eat away at Roy. He played 65 games that season and was limited to just three playoff appearances. The following year, he appeared in 47 contests before retiring.

He attempted to make a return with the Timberwolves in 2012-13, but played in just five games before succumbing to his pesky knees for good.

8. Greg Oden

The Blazers hoped to pair Greg Oden with Roy to form a dominant inside-out tandem. Unfortunately, injuries derailed the careers of both, and in Oden's case, it happened before his career really even began.

Oden was drafted first overall by Portland in 2007, but due to microfracture surgery, he missed his entire rookie campaign.

The big man made his NBA debut the next season, but he was limited to just 61 games because his knee kept acting up. When he was on the floor, he was effective, posting 8.9 points and 7.0 boards a night, but it was becoming increasingly obvious that the 7-footer was in danger.

NBA, Greg Oden

Then, in December of the 2009-10 campaign, Oden suffered another severe knee injury after 21 games. He would never play another game for the Blazers and missed three straight seasons between 2010-11 and 2012-13 before attempting to make an NBA return with the Miami Heat in 2013-14. He participated in a mere 23 games. That marked the end of his NBA tenure.

Oden was compared by some to David Robinson, but he never got the chance to establish himself.

7. Shawn Kemp

While most players on this list were hamstrung by injuries, Shawn Kemp failed to maximize his potential as a result of laziness and off-the-court issues.

At his peak with the Seattle SuperSonics, Kemp was a monster. He was the most violent in-game dunker this league has ever seen. He could shoot from mid-range. He could run the floor. He could handle the ball like a guard. He could pass. He could rebound. He could guard multiple positions.

Shawn Kemp
Andrew D. Bernstein/NBAE via Getty Images

Kemp was so talented that he unquestionably had the potential to become one of the top five power forwards ever, and he made that very clear during the 1995-96 campaign when, along with Gary Payton, he led the Sonics to a Finals appearance before falling to Michael Jordan's Chicago Bulls.

But then things took a turn for the worse.

Whether it was drug and alcohol abuse or his weight ballooning, Kemp's career nosedived not long after he left Seattle in 1997.

We'll never know just how good Kemp could have been had he kept his head on straight.

6. Derrick Rose

It feels like ages ago that Derrick Rose was one of the most explosive players in basketball, but it was real.

Three seasons into his NBA career, the Bulls' No. 1 overall pick was on top of the world. He averaged 25.0 points and 7.7 assists per game en route to winning the MVP award, becoming the youngest MVP in league history in the process. He led Chicago to the league's best record and an Eastern Conference Finals appearance. He was the favorite player of many kids around the country.

Derrick Rose, Bulls

But then, in his fourth year, injuries limited Rose to just 39 games in a lockout-shortened 66-game campaign, and he tore his ACL in Game 1 of the playoffs.

It was then a downward spiral for Rose, who has regularly battled knee injuries and other issues. He is still in the NBA and remains a solid bench scorer for the Detroit Pistons, but his dominant days are far behind him.

5. Penny Hardaway

Before there was Shaq and Kobe, there was Shaq and Penny.

Shaquille O'Neal and Penny Hardaway formed a fantastic duo with the Orlando Magic during the early stages of their career, making it all the way to the Finals in 1995 before being swept by Olajuwon and the Rockets.

Hardaway was an incredibly unique player early on as a 6-foot-7 point guard who could get buckets at will while also facilitating the offense and playing terrific defense.

Penny Hardaway, Michael Jordan, Magic, Bulls, NBA

But during his fourth season (what is it with Year 4?) in 1996-97, Penny was limited to just 59 games because of injury, and the following year, he appeared in just 19 contests.

That basically marked the end of Hardaway's run. He averaged over 20 points per game three times in his first four seasons, but after the injuries sabotaged him, just registering double figures became tough.

Hardaway played 14 seasons in total, posting 15.2 points and 5.0 assists per game.

4. Grant Hill

Grant Hill was basically LeBron James before LeBron James.

During his time with the Pistons in the mid '90s, Hill was regularly averaging in the neighborhood of 21/9/7. In fact, in 1999-00, he racked up 25.8 points per game.

Hill seemed destined to become one of the NBA's all-time greats, and when he signed with Orlando in free agency to team up with Tracy McGrady during the summer of 2000, fans were buzzing.

Grant Hill, Tracy McGrady, Magic, NBA

But an ankle injury limited Hill to just four games that ensuing season, and from that point on, he dealt with nagging injury issues throughout the remainder of his NBA career.

He was able to carve out a niche as a solid role player for the Phoenix Suns later on and actually remained healthy in the latter stages of his career, but that version of Hill was not even close to the version we all saw in Detroit.

3. Bill Walton

When you talk about the best collegiate basketball players of all time, you have to include Bill Walton in the discussion.

The big man was a monster at UCLA, parlaying his NCAA success into becoming the No. 1 overall pick by the Blazers in 1974.

Walton displayed his dominance very early on, propelling Portland to an NBA championship in his third season after averaging 18.6 points, 14.4 rebounds and 3.2 blocks per game. He won Finals MVP that year.

The following season, he won MVP.

Bill Walton, Blazers, NBA

However, injuries were derailing Walton all along, as congenitally bad feet consumed him very early in his career. As a matter of fact, he missed three whole seasons in the middle of his career due to foot issues.

Walton would later add another title with the Celtics in 1986, but by that point, he was a bench player. A darn good one for sure, but he was not even a shell of his former self.

Had he not had all of those injury problems, Walton probably would have been one of the greatest centers ever.

2. Reggie Lewis

Reggie Lewis seemed primed to take the torch from Larry Bird in Boston.

A first-round pick of the Celtics in 1987, Walton became an integral part of the C's as soon as his second season, averaging 18.5 points per game.

By his fifth year, Lewis had become the alpha male with Bird fading due to his back issues, registering 20.8 points per game and making an All-Star appearance. He posted nearly identical numbers the following year.

But then, during the 1993 playoffs, Lewis collapsed during a game against the Charlotte Hornets. It was revealed that Lewis had a serious heart issue, and that ensuing summer, Lewis tragically perished during an offseason practice.

Lewis appeared to be on his way to becoming a dominant wing scorer, but his career was horrifically cut short.

1. Len Bias

Another Celtics player, but this time, he didn't even make it on to the court for a game.

Len Bias was taken second overall by Boston in 1986 after an outstanding career at the University of Maryland. Boston was preparing to put Bias alongside of Bird, Kevin McHale and Robert Parish to form an unstoppable foursome.

But it wasn't to be.

Just two days after Bias was drafted by the C's, he tragically died from a drug overdose.

Bias was so athletically gifted that he was even compared to MJ when he was drafted. He was a 6-foot-8 freak capable of doing just about everything on the basketball court.

Sadly, we will never know just how great Bias could have become.