There is a point to be made here about depth and dropoff as well. There are teams who can lose a major part of their offense and be ok, because they have other guys who can make plays in one form or another. But then there are squads who can lose one player - even if they player isn't "that good" - and that loss can really sink a squad. Especially if that player is a defensive stopper, or other specialist.
I would say losing Malik Boothe would be really bad for the team. though Stith could technically fill in. After seeing Quincy handle... I would stop there unless he drastically improves his handle.
Sean Evans is second, but I say this without knowing what the team will look like next year or how they develop in the offseason. But Evans is the most likely interior player to log heavy minutes - he (surprisingly) is the least likely to pick up fouls defending inside. Justin might be able to play 35 minutes and defend well, but he didn't last year; and Rob Thomas needed more stamina.
Also, Evans is a huge part of the one thing the offense does well - bang on the offensive glass. It would be nice (and more effective) if it was Justin on the offensive boards... but that's the skill Evans brings to the table - hustle and offensive rebounding. Someone else could pick up defensive rebounds in the rotation (if Kennedy was hurt); Coker if he could stay out of foul trouble, Brownlee (hopefully)... but no one else has the size and quickness that Evans has.