All weaknesses can be improved on.. His positioning needs tweaking, meaning he needs to press and hunt for the ball with intelligence. Passing through the lines requires more bravery and movement of teammates, again... Ten Hag needs to get the team syncing. I believe Ugarte reads the game well, technically strong, as he can run with the ball and evade opponents. He is not a classic sitting midfielder or a playmaker, but I think midfielders should be good at more of those things than not. He will need to step up another level here if he arrives, but the fundamentals are there, and a good coach can improve his game.
Secondly, people need to stop thinking of one single player as a savior of the team. The question we ought to ask is, what are his strengths? What does he bring to elevate the side? United should always be out there adding more players with quality into that central midfield.