A medical would all take care of that, and the club could always speak to a specialist before. At 21 a knee injury is hardly going to keep you out of playing top level football, ligament damage is different.
If you are 35 and you have played 60 games a season for 14 years a knee injury is catastrophic, for Upamecano I doubt it would be.
Good thing for United versus Arsenal is, the United scouts are usually pretty good with their judgements when signing players, the Arsenal scouts usually sign blindly and it never works out.