*facepalm*
He's not suited for 6 or 8. His qualities there are mediocre at best. He's a 10, however not a creative one. His qualities in and around the box are great though. I understand these type of players are not very common as they are not used very often. Teams play a creative #10 or without a 10.
The reason why Donny hasn't been that great at 10 for United is because he has been made to play as a creative 10. He's not a Bruno replacement, he has total different qualities. When Bruno is not playing, you put Donny there and you expect him to do exactly what Bruno is doing, you just don't understand the characteristics of the player. Donny never did those things when he did great at Ajax. Its not like he played like Bruno did when Donny was back at Ajax and now at PL level he can't do it. He never played like Bruno in the first place. He's a totally different player, to ve used in a different setup.
Donny has great qualities in and around the box and therefor 10 is by far his best position. He needs however creative players around him that will find him in those area's. He will be at the right place at the right time, capitalize on the opportunities and find the right solutions there. However if you're waiting for him to be the playmaker setting up all the attackers with great passes fdom the half way line, you're going to wait a very long time. He will do so occasionally, but it's never been his game to always give that pass.
At 6 or 8, sure Donny might be a decent player, but he'll never be the significant improvement over McFred that you're looking for.