While I am concerned about JO missing games, I think people equate his fragility with a decline in talent, and I don't think that had happened nearly as much as some people still to think.
I truly believe that when you count both ends of the court, he can still be a top 10 center in the league. And I'll go a step further and say that he can have the best year this year that any Celtic center of the new Big Three Era if he is miraculously relatively healthy.
Why? Because he is defensively about Perk's equal, not quite matching Perk in 1 on 1 defense, but vastly surpassing Perk with his shot blocking. He's obviously miles ahead of what Shaq gave is defensively last year. Offensively, people overlook his midrange game, which is quite good and something that will be deadly if he is left alone. And while he's not quite Shaq around the basket, he's a much better finisher than Perk. And we all saw last year how many easy baskets there are to NE had playing with the Core Four.
Again, missing games is still very much a concern, but I think when on the court JO can still play at a very high level.