I personally think this is actually more important to get right than Sancho. I've no idea what the club were thinking throwing 3 years at Matic, but aside from a handful of games, he's not looked at the top level for at least 2 seasons now. He's a lovely passer of the ball if left completely alone and he's kind of useful at defending corners, but he's really not a defensive-minded midfielder. He's also ponderous and can be a real liability to the press.
We really need a dynamic, technical, defensive specialist if we're going to play with Pogba and Fernandes. Go on scouts/fm legends - who is the next Kante? That type of player. Someone who has energy to burn, is happy to sit but also capable of quickly moving the ball along.