The way Onana has been playing, we seriously need a capable No.2 before he costs us another CL group exit. Might as well start with Vitek. Make him our cup keeper. As good as the loans are, nothing will prepare him more mentally to play for United like playing for United.
We have quite a lot of cup games this year: the Europa League group stages, the Carling cup or whatever it's called these days and early games of the FA cup against lower tier opposition should all be his games to get his feet wet and see if he is ready to make the step up.
He was rated higher than Kovar, and considering how well he's been doing, might as well give him a shot then. And if Onana decides to let in his usual 4-5 howlers in a row, we can bench him and stick Vitek between the sticks. ETH has shown that he can drop a GK midseason if he has to at Ajax, so I see no reason as to why not here.
If not, let's just sell him and not waste our time or his development and sign a competent No.2