Doc Rivers and the Philadelphia Sixers have a daunting task on their hands as they gear up for the start of the regular season. Ben Simmons continues his holdout as he eyes a trade and has shown no signs of a possible change of heart. It is now on Rivers and his staff to fill the void left by Simmons with the players in house.

The main area that will need addressing is defense. Over the past two seasons, Simmons has cemented himself among the top defenders in the NBA right now. His ability to defend all five positions at such a high level is what landed him second in Defensive Player of The Year voting last year.

Defenders with the versatility of Simmons are extremely rare and almost impossible to replace. Following practice on Sunday, Rivers gave a small glimpse into how losing Simmons affects the team's defensive game plan.

“It makes us smaller. What Ben [Simmons] did for us, we could pick one through four before the game, and we would look at who is the best player one through four, and put on them. We don’t have anyone else that can do that, and that was a huge luxury,” said Rivers.

Cementing themselves as a defensive powerhouse has been the main catalyst of the Sixers being in the conversation to compete for a title in recent years. Between the perimeter versatility of Simmons and Joel Embiid's rim protection, Philly arguably had the best defensive duo in the league.

Similar to the offensive side of the ball, no one player is going to replace what Simmons brings to the table. Players understand they will have to step up collectively to soften the blow of losing the three-time All-Star. Danny Green opened up on this team mindset during his media availability Sunday afternoon.

“We have to do it collectively as a group. Obviously, it’s going to be different with matchups because we can’t just put me or Matisse [Thybulle] on somebody when there is bigger guys out there. We can’t have Seth [Curry] on a bigger guard or Tyrese [Maxey] on a bigger guard or Shake [Milton]. It is going to be all of us trying to collectively stop those stars. You can’t replace what he did on the defensive end with just one person,” Green said.

Not having Simmons is certainly a huge blow defensively, but luckily for the Sixers, the personnel that can fill the void. Green is still an above-average defender, and Matisse Thybulle is fresh off being named to an All-Defensive team. The way they attack that side of the floor might change, but the Sixers still have the pieces to remain near the top of the league in terms of defense.

With a player like Joel Embiid anchoring the defense, the Sixers will always be above average on that side of the floor. That said, how they adapt to life without Simmons' all-world perimeter defense will be one storyline to watch in the early weeks of the regular season.