Aaaand this kind of attitude is exactly what's gotten us in this 10 year mess.
Stop thinking of players in terms of monetary assets. It's why we're shit at football, we hang on to dross like Sancho, Martial, Jones, Rashford, Baily, Anthony, Pogba and countless other players because we paid a lot of mone for them "one day they'll come good".
The fact is some players don't have the mental toughness to play for United, they cant take the pressure. It doesn't matter if they go out on loan and set the world alight if they wilt under the social media/ press/ old trafford lights, they are worthless to us on the pitch and should never be judged against their price tag.
Sell him while his stock is high. He did nothing for us when he was fit and was given countless chances. If he comes back he'll do nothing again, because guess what? He can't hack it here. It's a different beast to dortmund.