Players with peak Matić's characteristics are quite rare — titanic proportions, decent ability on the ball, good reading of the game, a bit of attacking mobility from deep — and the best replacement joined Atlético in the summer (Rodri). Though we didn't really get peak Matić for an extended period of time, so there's that. You have to consider any incumbent manager's general midfield plan as well — if say Zidane joins United and puts a possession oriented 8 ahead of the 6, you'll be better off targeting a defensive midfielder who can operate as a water carrier and a terrier (think Casemiro and how important he was in Madrid'd structure), as opposed to Rúben Neves (who's more of a holder and playmaker). Someone who is combative and committed, but also tactically intelligent and can routinely break up opposition moves in front of the defense, like maybe Ascacíbar (diminutive, but kinda comparable to Mascherano in terms of characteristics)...