I've always rated Matic. Even when he was poor and people were saying we need to sell him. It was evident he was over played by Mourinho (again like the end of the title winning season with Chelsea going into the next).
Technicaly, positionally, reading of the game, passing, ability to hold and move the ball, tight space control etc he is brilliant imo. However, the major issue is not him but the midfield partnership of Pogba and Bruno, that passive bag of piss Lindelof and to an extent Ole's tactic/coaching.
Right now a well rested Matic is more than apt against most opposition. My problem is that the above conditions may means there is a greater area of defensive coverage, space between defence and midfield and a tactical lapse, which makes any third midfielder potentially suspect.
In 5-15 yard spaces, hands down Matic is superior. It's the 15-30 yards where I just think Fred's tenacity and overall skillsket (whilst not as refined, is as rounded but good in other aspects) may complement the others.
I.e In a well drilled unit with everyone taking equal and clear responsibility in spacing, pressing, shape etc, it's Matic. In this organised chaos of well managed players with freedom to express but light on details, I'd go for Fred.
Lindelof can still do one though. Hopefully Bailly gets a chance.