We may blame Ten Hag's, he's in control and responsible for everything. The current squad cannot do anything for Manchester United, they must sign players during the transfer window. Consider Manchester United's defense line, Varane can no longer defend and there is no one to replace him at the moment. Even in their last game against Liverpool, Manchester United could not have equalized, if Klopp had not removed his top players.
Manchester United inconsistency is from the beginning of this season because they have been delivering poor results so far. Only a few of their games are good, which is not much. Further, the issue of Ten Hag's management is not from the players that are injured, because it is not only Manchester United's players that are injured; there are some clubs whose players are injured and they are still doing well in their matches. So to me, the issue of Ten Hag's management skills is that he did not have management skills; he doesn't know how to use his key players when they have a match, which is why they are still performing poorly.
However, if you look at Liverpool, you will see that some of their players are still injured, and they are performing well with the remaining players anytime they have a match, so for Ten Hag, I think he should put in more effort to lift up the squad.