The issue with Matic is that he cannot play a large number of games consecutively; he can play some, which is contrary to belief, but not enough to be our first team defensive midfielder. Still, he has deteriorated from his peak at Chelsea, so he does need replacing himself. Hopefully, this transfer window will see it done.
McTominay may have the strength and height desired for the role, but he does not have the passing. His pass completion is only 79.4% in the Premier League, which is nowhere near good enough. It is not like he is constantly trying to break the lines with defence-splitting passes, but just simple side-rolled ones. He gives it away far too often. Either he is easily pressed or just is a poor passer - in Europa League his passing % is 88.1%, but nosedives against much better opposition. (Fred's only drops from 90.4% in Europa League to 87.2% in the Premier League).
Defensively, McTominay is better than Fred in certain areas: heading and outmuscling opponents (though, he was knocked off the ball a few times by Lamela yesterday), but not in others. Fred makes more tackles, blocks, and interceptions per game. This also brings into account McTominay's positioning, which can be dodgy at times.
Fred does not have the strength nor height but is a far better passer than McTominay. His average pass completion is 87.2% with more long balls, more key passes, more crosses, and more passes in general when compared to McTominay. It shows that Fred is trying more complicated passes and is still completing a far higher percentage. Fred also seems to have limitless energy and is quick; he has the fastest recorded speed for Manutd this season. Fred is better at positioning, but he can be outmuscled at times.
I think if you want to challenge for titles, you cannot afford to have a player that cannot pass. For comparison, Fernandinho' pass completion last season was 87% with more long balls and passes than McTominay (Used last season as he has played as a defender this season); Fabinho's is at 85.4% with more long balls and more passes. In this aspect, McTominay is nowhere near up to standard. Matic has seen a drop himself; at his peak, he used to be a 86% to 88% pass completion player, but now is 84%.
Matic is the best as he has the most assets needed for the role, but only for the short term. We need a defensive midfielder if we are going to play Pogba and Fernandes next season. Matic, Fred, and McTominay are not the answer. Matic, however, is for the remainder of this season.