A good reserve? With his performance like that for Manchester United I think being a reserve player is not really worth it either. One of the reasons Manchester United don't sell is that they are afraid of losing too much, but rather than just throwing money away on a salary that is not comparable to his performance I think 20 million is still very worth it.
Don't forget the tradition for the England national team, whoever the players are will become players who are often defended by the club even though the player's performance is bad, including Maguire himself has been trusted as a player for the England national team for a long time. So I'm not surprised that Man United still doesn't let him go, especially at such a cheap price. However, there is a slight difference in principle, Erik Ten Hag actually dared to take decisive action, namely selling Maguire, which the Man United coach had never done.
And with regards to Manchester United, the team is in need of a striker because, even in their preseason game, the only position that is lacking is that of a striker. Manchester United likely needs a striker by the start of the league, but from what I can tell, it will be very challenging for the team to find a good striker because all of the top popular striker players that we are aware of are on the market, many teams are vying for their services, and they are about to sign them, Manchester United must take action quickly before the league begins because they are also participating in the champions league, which is not the same as the other leagues you could think of as minor leagues.
More precisely, Man United needs a striker with sharp instincts, not only capable of good finishing but also very commensurate individual skills. Therefore, Erik Ten Hag's big hopes need to be fulfilled by Rasmus Hojlund. He will be different from Rashford style of play which tends to be unstable in terms of execution and often loses control of the ball, so Sancho, Antony and Garnacho also need to show balance in supplying the ball to the midfield.