So, he had a history of back problems and it's clear that they're working on his back in the corner from round 6 or 7 (EDIT: after rd 7) so it's certainly not made up from nothing. BUT - I'm always sceptical of injuries having effected a performance much, because the adrenaline makes fighters able to work through most things. Broken hands are one typical thing that fighters just go on with. And in this fight Floyd said the injury happened some time during it and after round 7 is the first sign we see of it in his corner, and Ali was playing with him the whole fight. It wasn't like it was fairly competitive and then Floyd just dropped off. Ali was bossing him every second of every round. The injury wasn't decisive in any way. It was one of the three, four best versions of Ali imo and Floyd was just out of his depth.
I will say, in Floyd Patterson's defense, that boxers who often employ his general style (with individual variations, of course) often eventually have back problems, i.e. Jack Dempsey, Rocky Marciano, Mike Tyson, etc.
He did have chronic back problems. Never heard it brought up as an excuse for any other fight of his, though. EDIT: But his problems in this fight was worse than usual, though, as you can see his corner working on his back after round 7.
It was noted by ringside commentary before the knockdown round. Their rematch demonstrated that his bad back was not pivotal to the outcome of their first bout, but without it, Patterson might've reached the final bell. Floyd was very obviously badly outmatched from the beginning in 1966.
Rocky Marciano had back problems. Floyd had back problems. We should have gotten Rocky vs. Patterson — promote it as who can be the king of Brokeback Mountain!
Rocky didn't use it as an excuse, he turned down back surgery which almost killed JFK, and is still a significant procedure over 70 years later. (Charley Goldman did a great job applying the stretching and tugging exercises on Rocky's back which were the alternative to an operation. Goldman wasn't merely his trainer, but his physical therapist.)