The Boston Celtics lost Kyrie Irving, Al Horford, Marcus Morris, and Terry Rozier to free agency, and they're 25-10 this season. This is yet another testament to head coach Brad Stevens' system.

Back in training camp the Celtics looked like a playoff team, but little more given the aforementioned departures. Irving was the driving force of their offense; Horford is one of the NBA's most well-rounded centers; Morris is a steady two-way player; and Rozier was a spark plug off the bench.

This was a unit that advanced to the second round of the playoffs in back-to-back seasons. Granted, one could argue Kemba Walker replacing Irving was a wash, though these voids weren't totally filled. Sure, Enes Kanter has a reliable post-up game and is one of the most efficient rebounders in the NBA, but he struggles defensively and isn't the outside shooting threat he used to be.

Any unforeseen playoff run or 50-plus win season was going to hinge on severe internal growth. Well, that has happened, and the Celtics' MO hasn't changed; one could argue it has never been stronger.

This is an elite defensive team. They swarm ball handlers, defend the perimeter well, and are adept at switching covers on the fly. The Celtics went into Wednesday first in the NBA in opponent points per game (103.5), third in defensive net rating (103.8), and fourth in opponent field goal percentage (43.7 percent).

Offensively, it's more of the same: an elite point guard running the show, a handful of scoring wings, and well-versed center play. Their wings and forwards are the big story, though, more specifically Jaylen Brown.

Over the first three seasons of his career, Brown was a reliable defender who was an athletic scorer but never an integral source of offense. This season he has been an irreplaceable scorer. Averaging a career-high 20.5 points per game on 50.7 percent shooting from the field and 38.7 percent shooting from beyond the arc, Brown has been one of the most improved players in the league. He looks comfortable with the ball in his hands and is taking timely shots while still moving the ball and not being a ball hog, so to speak.

Meanwhile, Jayson Tatum has taken the next step from a production standpoint, averaging a career-high 20.9 points per game. He looks like the player the league expected him to be last season.

From a production standpoint, the Celtics are in the middle of the pack offensively, as they went into Wednesday 14th in the NBA in points per game (110.9). With that said, it doesn't tell the whole story. Rather than force perimeter shots, the Celtics move the ball, get the ball inside for easy buckets, and are an efficient unit. They went into Wednesday seventh in the NBA in offensive net rating (111.2).

Lockdown defense, working for easier buckets, and having players who execute in isolation; this is vintage Boston Celtics under Brad Stevens.

The principles are the same. The production is the same. The product is the same.

Stevens is getting the most out of every player in his rotation. Brown and Tatum are tapping into their potential; Walker has been his potent self, finishing with style off the dribble and getting others involved; and Gordon Hayward looks like his old self, averaging 16.7 points per game.

Marcus Smart has been his pesty-self, defensively; Kanter and Daniel Theis are finishing inside and hitting the boards. When others get the nod from Stevens, they answer the call, playing tight defense and moving the ball.

The best coaches are the ones who get the most out of their players. Nick Nurse has a Toronto Raptors team that was widely viewed as a team that could sneak into the playoffs, but do little more back in contention. Last season Doc Rivers coached a team that traded its top scorer, Tobias Harris, before the NBA trade deadline to 48 wins while pushing the otherworldly Golden State Warriors to six games in the first round of the playoffs.

Two seasons ago, Gregg Popovich coached a San Antonio Spurs team playing without its franchise player, Kawhi Leonard, to the playoffs; in that same season, Stevens coached a Celtics team that lost Hayward for the season due to a gruesome leg injury on opening night to a 22-4 start. Then he saw that same team take the LeBron James-led Cleveland Cavaliers to seven games in the Eastern Conference Finals with Irving sidelined.

Who thought the Celtics would be the three seed in the Eastern Conference in the new year with the free agent losses and significant point guard change?

The Celtics are at their best when they make it a halfcourt game. They find ways to create shots, suffocate opponents on the defensive end, and pounce in transition with Brown and Tatum. They make teams feel comfortable, and then ambush them in fastbreak with their concealed athleticism.

There's no more questions about a franchise player leaving in free agency for the Celtics, and no one has publicly expressed disgust with their teammates. The Walker era has been a breath of fresh air for the Celtics; the point guard and his head coach seem to be on the same page.

The Celtics are doing what they should've been doing last season; playing at a championship level.

Will they overcome the likes of the Raptors, Milwaukee Bucks, Philadelphia 76ers, and Miami Heat in the spring? Inevitably, they'll endure a losing skid that invites concern. One thing they can lean on when times are rough or their backs are against the wall in the playoffs is continuity.

A considerable chunk of this team has been together for the last three-to-five years. They know each other's tendencies and have been through promising highs and discouraging lows.

The Celtics remain an NBA Finals contender. Of course, players picking up the slack is a vital reason for their contender status, but Stevens' coaching is just as crucial. Yet again, the Celtics are finding a way to stay in the mix.