He was injured in late February. That sort of injury usually takes a little less than a year to fully recover from. With the team in rebuild mode, there's no way they'll rush him back.
I don't expect him to miss half the season, but I doubt he'll be playing before December, and even once he's back I think he'll come back relatively slowly and they'll probably hold him out of games anytime he has even a minor injury.
I'd expect Rondo to play about 50-55 games next season.