Manchester United's main problem doesn't seem to be entirely about players for the 2021/2022 season. They just didn't have the right coach and strategy to deal with all the available players, it failed because I think the Manchester United team have enough players compared to Arsenal, Tottenham and some others.
They have issues from transfer strategy, coach tactic to player performance and mentality. Transfer strategy is bad or does not exist at all. Coach position changes three times in a single season. Coach tactics is undefined and that affect players too. Players themselves are very weak mentally and their performance are worse than what they can do usually.
Many talented players at Manchester United, but the coach did not manage to maximize his ability so that the results were really unsatisfactory. They need improvement, maybe some players will be brought in and hopefully improve the mentality of the team and the performance.
The coach position now belongs to Ten Hag and he is the biggest hope for Manchester United now. If he can plan a transfer strategy well, the club can be improved. Bad and unused players should be sold out, new players should be the ones are relevant with Ten Hag's tactic and club plans.