It isn't an excuse for having an unsuccessful season. It is a reason for it. 
Then what do you think bigger names than Erik ten Hag would be able to do in the same situation? Would they fight for the title anyway despite having a big number of injuries?
When it comes to your Manchester City example... You are missing a point here. There is an enormous gap between the squad qualities of Manchester City and Manchester United. Even when they didn't have De Bruyne, Guardiola didn't face big problems. Because he still had a squad full of stars to make up for the weaknesses.
But it isn't the same for the Red Devils. They already had to improve their squad and those injuries made things much worse.
It cannot be denied that injuries were the main reason the Manchester United team slumped last season, but this cannot be prolonged and Ten Hag has the responsibility to overcome this because the Manchester United team has been completely entrusted to Ten Hag to build it, from the start. Ten Hag's arrival has pretty much fulfilled his desire to recruit players, but the team's performance is still far from expectations. Then what do you think bigger names than Erik ten Hag would be able to do in the same situation? Would they fight for the title anyway despite having a big number of injuries?When it comes to your Manchester City example... You are missing a point here. There is an enormous gap between the squad qualities of Manchester City and Manchester United. Even when they didn't have De Bruyne, Guardiola didn't face big problems. Because he still had a squad full of stars to make up for the weaknesses.
But it isn't the same for the Red Devils. They already had to improve their squad and those injuries made things much worse.
The Manchester United team has only been built for a few seasons by Ten Hag and it is very different from Manchester City, I don't think it can be compared and in the future I still believe that if Ten Hag continues to make improvements and cover up the mistakes that have occurred then it is very likely that the Manchester team will United will be perfect again or match Manchester City's level for the title competition.