I am also quite curious why they are interested in this young player. But at least, he is considered talented, his position is only as a left-back. and at his very young age, Man United dare to take him with a fee of $5m plus add-ons. whereas for now, his market value is € 1.00m according to the transfer market data. and the reason Man Unite chose Diego Leon is because they believe that the left-back player can be a long-term asset for the strength of Man United's back line.
Manchester United needs a defender and maybe they see a match for his playing style and his young age also has the opportunity to develop into a mainstay defender in the future. Talking about opportunities, it might be big for him to develop because the defenders owned by Manchester United are now in an uncertain condition due to injury factors. The market price for the player is also not too high so Manchester United does not need to spend a lot of money. I also saw some video clips about him and indeed his defensive ability and running power are quite promising and maybe this will be a long-term prospect for Manchester United.