Let's be realistic here, he is a really good player with a lot of potential but he isn't better than our first team players right now. He'd get the league cup games here and a few games here and there filling in when there are injuries/suspensions. It looks like Valencia see him as a key player for them so he'll likely play every week, it'll be a good move for all parties involved imo.
Very much this.
Who would you play him ahead of? Mkhi? Martial? Rashford? It certainly wouldn't be Pogba or Matic. He needs to play as a first choice player in a decent team in a good league or he won't go anywhere. You don't suddenly turn into a first team player by spending a season on the bench and getting the odd Carabao Cup game. Even if we presume that he would get a game in any midfield position, we already have Pogba, Matic, Carrick, Herrera, Fellaini, Mata, Mkhi, Young, Lingard, Martial and Rashford vying for 5 spots in the team.
This isn't Football Manager. You can't just play a second string XI whenever you have a home game against Burnley or Brighton.