Perhaps they are delaying his come-back because this year's medical staff is more attuned to a player's injury status. One that waits to make sure a player has healed enough so as not to reinjure himself.
We've already seen them keep Marcus Smart out for a couple games in what appeared to be a minor ankle injury. In previous years he likely would have played through the pain.
What used to bother me about the previous training staff was there were times when players were cleared to play when they were in obvious pain. What baffled me even more was Brad seemed to be oblivious to their ineptness and kept putting these injured players in the games. Hopefully, the previous staff didn't carelessly put Isaiah's career in jeopardy by clearing him to play while he had a serious hip injury.
Might also be that management has instructed this medical team to be more conservative and proactive in their treatment approaches.
And, of course, Marcus could be seriously hurt and any medical staff would have held him back.