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.
Yes, Amorim as a manager can see the potential that exists on Diego Leon. he wants to mold players from the beginning, and make him a great player who can be a valuable asset for the future. I certainly don't doubt him because he did it on Sporting. As we know Gyokeres are not a great player before, but after Amorim recruit him, and mold him to best attacker, then he has a high marketability right now. We can look the comparison now, Sporting on new coach was drop drastically, Gyokeres become a barren, and not as aggressive as when trained by amorim.
So for this season, Amorim has a target to build a team first, Then after the team is solidly formed, he will start with confidence in the next season.