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.
In the past, Manchester United failed to scout and sign younger talents but rather went for overrated players which didn't turn out well, but i think this new Sporting management of the club led by Sir Jim Ratcliffe and his crew are beginning to figure out the errors made by the previous management (Glaziers) and seeking possible solution, so i believe that's why they're beginning to prioritize younger talents over made players that would be signed for a huge figure yet they won't make much impact to the team, it's a welcome development and highly needed concerning the transition stage they're currently in, Ruben Amorin would be pleased with this cause he needs younger talents who would hit the ground running and blend in immediately they come in for him to succeed as a manager here.