Its the right thing to do. The guy is way too young and way too talented to be going through so many ankle injuries in such a short time. What Pop should have done was sit him out for game 1 and 2 on top of the last game against the Rockets. By now the ankle would have probably been close to 100% and he would be starting tonight. Then again there is no way of knowing how severe the sprain is.
Seems like now is a lot better time than game 1/2. Had those games gone well and they stole a win from GS, they'd be in good shape (hell, they just about did just that in game 1). He deserved a chance to see how he'd play on the ankle, considering the context. Clearly he's in danger now that he tweaked it like 2 or 3 times in game 1 alone.
Not an easy decision regardless though, I think.