Manchester United only have 3 midfielders worthy of that description: Matic, McTominay and Fred.
Bruno, Pogba and VdB share some similarity to midfielders, but none of them actually are. If you wanted to play two of them at the same time, you'd need prime Makalele behind them to pull it off.
So, if you assume Bruno is a nailed on starter, you need to pick two out of the rest to sit behind...
Matic is too slow to cope with our high line. Pogba doesn't work because he's too careless in possession and too poor at pressing. VdB doesn't have the defensive awareness. And since we don't have a young Makalele in the team, the inclusion of the latter two is moot anyway.
So Fred+McTominay get in by default. Neither is top class on his own. But as a pair they work beautifully together. People on this forum love to talk about the 'double pivot' whilst misunderstanding it entails. Fred+McTominay actually are one. They're excellent at taking turn to sit and press. And because they share those burdens, it's much easier for them to pick the right positions at the right times on their sides of the pitch.
Matic makes a better DM in a midfield 3. Fred+McTominay make a better defensive pairing in double pivot. And whilst the latter option may not be the most creative, at least they make us more solid through the middle. That in turn allows Shaw/AWB/Bruno to play higher up the pitch. So it's swings and roundabouts.
A boxer can't go into a fight swinging big on every punch. He'll get knocked out before he lands one on his opponent. Sometimes a strong defence is the best way to get yourself into an advanced position. We already have enough weapons to cause problems when we do.
Fred+McTominay: they attac but they also protec