I wrote an elaborate piece on Donny when he just came to United, see below.. This still holds true for me.. (apart from my last suggestion to perhaps switch to a 442 diamond is not something I would suggest today anymore now that Sancho came..)
I wasnt able to see the match yesterday, but I read a few comments about him connecting well with Mata and that doesnt surprise me at all. I also read some comments stating he was kinda invisible on the pitch for a central midfielder and to be frank.. That also doesnt suprise me at all, looking at Donny's strenghts and looking at the line up yesterday... As someone who watches every Ajax match, I think three of Donny's major strenghts are the following.
1) Donny is a very good in combinations in tight spaces. In this he is a typical Ajax youth product, who is comfortable receiving the ball with little room, finding a close open man with one touch and moving immediately away again to receive the ball somewhere else and making room for other players. I read a couple times above that he was looking for 1-2's a lot and this is recognizable for me. His technique is not spectacular on the eye, but very effective and solid. Thats why I get the link with other technical gifted players close to him in the pitch, like Mata, and thats why I think he could play really well together with Bruno.
2) Donny finds spaces behind the opponents defense. When is isnt directly involved in the combination where the attack is being built up, he is a very good ball receiving midfielder. Somehow he is always 'there' and always 'open' in the final phase of an attack, whether to finish it of himself or to give the final pass to a teammate. Thats why with Ajax he worked extremely well with Tadic as a false number 9 who involved himself in the play and left spaces for Donny to dive into.
3) Donny gives you goals. He is not only good at finding spaces on the pitch and behind defenders, but when he receives it there he is very clinical. The goal against Palace was a good example of that. I think he has a very high conversion rate for changes in the box.
Donny is NOT a player who finds the creative solutions himself, with a pass between the lines (at Ajax that were Ziyech, Tadic, Promes, Neres) or by picking up the ball from defense and bringing it forward (at Ajax that were Frenkie, Blind, Schone). He is just not going to be that player for you.. You need other on your midfield to do so... So therefore seeing he was paired with Matic and Fred yesterday, I can understand is was very hard on the eyes, because you have no one to create creativity from midfield for you.. Ole should know this and not expect Donny to play like this..
If you start expecting Donny to be the creative brain in your team, United have completely wasted their money. However when used in his strenghts, I think he can be a very very good player for a world class team.
With Ajax this fit perfectly, because you had other players around him who did the things he's not world class in - finding the open spaces in creative manners - but he was used in his strenghts by Ajax playing combination football with other players comfortable on the ball and therewith creating spaces with quick combination football, ánd diving into the deep spaces left by Ziyech, Tadic and Neres in the attacking line.
How would this translate into United?
I think a midield with Pogba Bruno and Donny would never work on the highest level, as there is no balance. I think you always need a midfielder who is behind the ball 95% of the times. At Ajax that were Schone, Frenkie and last year Martinez. At United that could be Matic. If you play Bruno, Pogba ánd Donny you wouldnt use all of them in their strenghts as one of them would have to play in the #6 role, otherwise you risk getting your midfield overrun. I do think United however needs all of Donny, Bruno and Pogba on the pitch because all of them contain crucial qualities which no other current United players have. Donny bringhts depth and scoring ability off the ball, while also have the technical capabilities to link up with Bruno and Pogba. Bruno is the creative brain upfront and Pogba can be worldclass as a midfielder who brings the ball from back to front.
So íf you want to play them all, you need to switch strategy in my opinion, by either playing with Bruno as a false #9 in a 433 like situation, ór in a 442 diamond situation... Both are quite similiar actually). I think both could work extremely well as you will also be able to rotate the two attacking positions with Rashford, Greenwood and Martial. Remaining problem however is that in both scenario's you need proper wingbacks who can also provide some wide threats.