Sullinger got hurt when he landed on a foot doing basketball activities for Toronto. No guarantee he gets hurt in Boston since it wasn't an injury he had when he left Boston. Now sure, maybe he was destined to get hurt, but he wasn't hurt last year. Can't project things like that.
And my larger point is simply, by overpaying for Horford, Boston lost out on addressing actual needs of the team. Boston is a terrible rebounder team, still has no wing scorer, has no low post scorer, etc. Signing a beta to the contract of an alpha when you don't have an alpha is a recipe for disaster. For the money Boston paid Horford it could have re-signed Sullinger and brought in Biyombo and Hibbert. Or it could have signed Gerald Henderson, Speights, and Mozgov. Or signed Trevor Booker and Luol Deng. Or countless other combinations of players that would better address actual needs of the team i.e. rebounding, low post scoring, a deeper bench, etc.
Boston wasn't a contender and signing Horford doesn't make Boston a contender. Horford is 30 the rest of the core of the team is 27 or younger so he doesn't fit in age wise either.
His signing definitely signals what Ainge is thinking i.e. Ainge is going to trade assets to try and build a contender now. Ainge obviously hasn't found a trade he likes well enough yet, but before the start of next season Ainge will have made a trade using prime assets to acquire a prime player because Boston needs an alpha to compete now and the Horford signing signals that is the intent.