Is there any means of accurately predicting that a player will do well in a club? Buying players is just like gambling the outcome is unpredictable. We have seen players who were underperforming in a club doing well in a new club and a new coach. Apart from Mason Mount, other players that were mentioned are still young and have time to improve if they are well-used by the manager. It is also noteworthy that the problem Manchester United is facing is complex and it will take a gradual process for the club to return back to its past form..
There is no way to know this in advance, but I think that the player's desire and his current form should speak volumes. On the other hand, you can always try to negotiate a loan for this player and see how good he will be in a new team, as happened with Antony in Betis, and now Betis is thinking about signing a longer-term contract with him. But if the player is good, he will be bought immediately because the demand will not allow you to just rent him.
That isn't still a guarantee that the player will continue performing on his peak form. I could remember when Gnarcho came to United, he was playing pretty good and scoring goals in his first season. Suddenly, he became a shadow of himself that can no longer score goals like he did before. I begin to wonder if United is giving these players too much luxurious life that's making them not to put in their best.