None of that would change whether he's doing a good job as a DM in a 4-2-3-1
They play an incredibly important role for the balance of the team filling holes so other players can get forward while they are staying back. If they arent doing this it doesnt matter how many passes they make, the team suffers overall. If he makes 10 less passes but fills in for our fullbacks and makes a vital tackle on the edge of the box when a player known for shooting from range has a chance to get a strike on goal then hes played his role much better because nobody else is going to be doing those things but other players are going to make passes.
So he might do a good amount for a central midfielder overall, but hes not playing central midfield and we don't need someone playing central midfield more than someone to help protect the defence. That isnt where we're vulnerable. Its our defence and thats why we're playing a system with 2 DMs in the first place.
So to recap - our DMs dont get forward and support play much, dont get in front of the ball they stay behind it. They dont currently fill in for our fullbacks who get forward to provide width in moves and will be out of position when we're broken against. They tend to stay central trying to block the central pathways... But if you run at Fred you're more likely to go past him than lose the ball, which is an easy way to bypass that extra protection. Matic and McTominay wont engage so often so opponents wont be pressed as much and may have more time on the ball in the build up, but they'll also be in a better defensive position to help the CBs clean up in the danger areas where Fred wont be as he pressed taking himself out of that position.
Being too easy to go past is a big liability for him and almost defeats the purpose of him pressing because of the simple solution to take him on when he comes to you.
If he's not going to give that defensive solidarity you need from a DM in our formation then he needs to play further forward. And that role doesnt exist in a 4-2-3-1. He isnt creative enough for the front roles. Maybe he could be used to press from the AM role in very specific scenarios but Bruno Fernandes is our 2nd best presser so why would you use a player half as good on the ball?
Also, if we then say hes a box to box midfielder and play a formation where we actually play one, then he'd need to score a lot more often and help create some more chances than he does currently.