He's always looked crap at the 10, much better when played deeper.
He plays too much like a 2nd striker, too high up the pitch looking for long runs. Maybe with Martial as the 9 it would work better, but he just isn't athletic enough really.
I wouldn't mind see him play the Eriksen role in midfield when he needs rotating out, but then we have Mcfred competing also.
Unfortunately for Donny it's not really worked out. Probably better to go home.
To be honest, the problem with him playing deeper is that he really likes to play as a 2nd striker-type 10. In the Ajax success season he played as the 2nd striker 10 pretty much during the best games and excelled at it. But that was when he had Frenkie and Schøne do the creative work behind him and had a false winger/10 in Ziyech on the right wing. It worked especially well with Tadic in the false 9 role, so he could pretty much dash into the space left behind in the striker areas. Scored quite a few goals and was part of great attacks that way. All in all he was really good at that role and it seemed to be a great talent at 10 or maybe someone who could use those skills at 8 as well.
That summer Frenkie and Lasse left and Donny initially had to take one of their positions, due to the available options left. Seemed logical, when he debuted he played at 6 and 8 mostly. However, suddenly the power midfield was turned into a one man army (in I think Martinez in his debut season, who was later accompanied by Alvarez more successfully, before moving out of the squad and later in at CB position). Donny wanted to be important in exactly the same way he was in the season prior. He was too stubborn to follow his responsibilities as a 6/8 player and Ajax' midfield was really weak for a while until he eventually got retrained by ETH to be better at 8 when needed, but it was never as good as his 10-role in the 18/19 season.
What I'm trying to say is that I think you're correct in your observation of Donny playing as a 2nd striker when deployed as 10. But unfortunately he'll do that a lot when played as 6 or 8 as well and then it's even more of a problem. In all honesty I think he can still do that 10 role well, but it really depends on the overall balance of the squad. Theoretically Casemiro and Eriksen should work as the players behind him, but with the wingers and strikers at Utd right now (except maybe for Sancho and Martial, or possibly Bruno as a Ziyech-like winger), you're no so well off with Donny. Maybe it's worth it to check it out with Martial and Sancho around him, but if it doesn't work out without very specific setups and players around him, especially in the moment of growth that Utd finds itself in right now, it's not the time and place for Donny, in my opinion. People talked about Pogba needing very specific players around him to be unlocked or useful with his very specific set of skills, but somehow Donny is even more specific in his skillset.
Maybe he'll prove us wrong as he recovers and adapts more from his lack of fitness and to the team and gameplan, but it's difficult when he's shown so much to disappear from midfield when an 8 and is too attached to his shadow runs when played as 10 that I don't know if it will work for him here. As I said, I am curious about a setup with Martial and Sancho in play as well (if recovered and adapted respectively themselves), but we will have to wait and see if he stays past the winter window at all to be able to see that option in action. With those players he could fit better than Bruno, but it could be that the confidence levels of both himself and his teammates in him will be too low that it's simply a lost cause now. I don't think ETH would play him if the latter were the case, but it's getting harder and harder to believe in this redemption arc.
Perhaps, as you say, he could try to become a Eriksen-sub, but I think he'll be reliant of a more midfielder-type 10 than Bruno is, and probably a more playmaker type winger than Rashford and Antony are, too. Again, maybe he will still prove us wrong as happened before with other midfielders (here and elsewhere), but is this hope naive or rational? (And is there an option more worth of our hopes by now? - especially with so little time left before the world cup and following winter window...)