The point is that he is that good. This season, for example, he has 10GA playing for an awful United team, having been excluded for the last 5 games and having been played in all sorts of positions before then. He’s also apparently in woeful form and desperate to move.
In comparison, England’s boy wonder, Anthony Gordon, has 8 GA playing for a fabulously attacking Newcastle team, with full support from his manager, and with no off-field distractions.
Now, of course, it might be said that the circus around Rashford makes him unattractive or that there are mysterious training issues holding him back. However, it’s not hard to see how a top manager might see the possibility of harnessing his ability and giving their team some added impetus.