Rashford himself his willing to take a cut just to be in Barcelona.. For now keeping him isn’t that much of a big problem until his load period ends and he performs to the extent of Barcelona willing to bring him officially into the club then maybe the problem could arise but I believe by then there could be some improvement for the club at large and hopefully they could let go of some players [like Lewandowski maybe as his contract also ends during that period aswell] to get fresh ones.. All will largely depends on individual contributions anyways.
He knows that there's no place for him in Manchester United this season and since Barcelona offered him an opportunity he had to seize it that's why he accepted a pay cut to move over there immediately and save himself the embarrassment Garnacho, Antony, Sancho and Malacia are facing currently at the club, being left out of the squad included in the club's tour to U.S for the Preseason games. It's now left for him to prove himself worthy of being signed permanently so Barcelona would see reasons to make it possible immediately his loan contract expires, else he'll be sent back to United just like Aston Villa did to him.