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.
Lol, Manchester United have been open to let him go the moment they got a replacement for him. The club dont want him anymore, the coach dont think Onana will ever improve in the team. It's a clarification that they team has all lost hope on him, its up to him to face a new chapter in his career.
Manchester United should also be aware they have not found that good goalkeeper yet. Until their new GK is tested and proving, they shouldn't be so happy at relieving Onana from their squad.