This is a good move for Man United, as he can't be expected to replace De Gea. It's only right that Man United let him go if they want to make a change for the better, as Man United problem lies in their weak defense. Furthermore, even if he stays at Man United, the club won't necessarily use him, as Man United have already signed a new goalkeeper this summer, so competition for places is quite fierce.
Why sound like Onana was the only issue at Manchester United, that's not true because the cause it's a team problem involving both the management. It's normal for any player who leaves Manchester United to do better elsewhere, Onana leaving Manchester United is a redemption to himself and to his career. Manchester United has spoken less of Onana like he is the only issue, now he is out let's see how quickly they will evaluate the whole issue and sort out their crisis.