Another loan is one too many, he has always had the technical and passing ability to command at least a squad role for a club like us (kind of like how Gibson, Cleverley etc did for us) and now he has added the requisite physicality so whats needed is to play him and see where he lands. Can he be a starter, a contender or cover?
If I was the one planning United's summer spending spree, I would plan my signings with him in mind. Buy a prime DM, have Garner take Pogba's place in midfield and see how he does in contention with the likes of Fred and McT whilst also getting chances as the first DM off the bench in case of injuries, suspension or rotation for the new DM.
We aren't going to be winning or losing the PL because of Garner so playing him wont be a risk but the potential upside is big. The trick with young players is that you will never actually know with them until you give them a real chance.
Players like Trent didn't just come out polished, someone invested minutes, dropped points and a lot of anguish in them. The pressure at United is unique and we will bever get a comparable loan to guage so why not go for it, he won't be the first or last to succeed or fail.