Fred and McTominay is a proper double pivot in the truest sense of the term. They each take turns pressing and shielding. And at least they make us hard to play against, whilst forcing a battle to win the midfield.
Matic and Pogba are a flat midfield two. Matic gets asked to do all the dirty work, Pogba is relied upon to provide the creativity. The problem is that both have severe weaknesses for their roles (mobility, press resistance, consistency, pass selection, etc). Unfortunately, they're also a piece of piss for the opposition to pick the ball off, and you could drive a bus through the gaps they leave between them.
At the end of the day, you can't just replace the former with the latter and expect a like-for-like system. It's the equivalent of swapping low grade diesel for high octane petrol in your car. One might be better than the other on paper. But your engine can only run on one of them.
The double pivot of Fred and McTominay is the reason we had any success post-Bruno. It the pairing we need to go back to. We were doomed the moment Pogba regained fitness, despite him obviously be more talented than the rest.
So although United may not be perfect when Fred and McTominay play, you can actually measure their solidity by the relatively small number chances they let through the middle. Just watch them and compare to the alternative.
We only have two options to avoid a complete disaster for the rest of the season imo.
a. 5-3-2 (okay, it's not Fred and McTominay, but it does pack central midfield and central defence to give the same benefit)
Rashford-------Martial
Pogba--VdB--Bruno
Telles------------------------Williams
Shaw--Maguire--Bailly*
DDG
*maybe McTominay/AWB for Bailly
b. 4-2-2-2
Rashford--Martial
Pogba---------------Bruno
Fred---McTominay
Telles------------------------------Williams
Maguire--Bailly
DDG