Mount didn't have an "injury prone record" before United. He had a pelvic injury that kept him out from February until May, which happened to be right before we signed him. Other than that, his only notable injury ever was in 2018/19 when he was a teenager at Derby.
While it's entirely fair to be disappointed in his time at United, let's not invent alternate realities to make it sound like the manager was insane to sign him because it was obvious that it would turn out this way. That's just daft. He was by no means an injury-prone player. In all his years at Chelsea, he had one single injury that was more than an inconsequential niggle, and that was an impact injury caused by a collision. Those are not taken into account when judging a player's propensity for injuries, for reasons that are hopefully obvious.
Incidentally, here's James Maddison's injury record. You know, the player you think we should have signed instead of the supposedly injury-plagued Mount. Note that it has a page 2. And they're all knee, ankle and hip injuries--you know, the kind that tend to say a lot about how injury-prone a player is, as opposed to collision injuries which are just bad luck.
Reminds me of the discussions about a potential Osimhen transfer a couple of years back, where some people complained about his injury record. I looked it up--he'd had two noteworthy injuries in his career: one where a player slammed into him and dislocated his shoulder, and one where he fractured his eye socket after someone headbutted him. How people can consider a player like that "injury-prone" is beyond me. Don't know whether it's stupidity or just being too lazy to look things up, but it must be one or the other.