There's a lot of things to consider here, such as:
What formation do we play
What type of player do we want in midfield
What kind of role will the signing perform
If we're after a pure destroyer/ball winner, we can look at the likes of Danilo Pereira, William Carvalho or Steven N'zonzi. They are all big units who are defensively solid and aren't flashy with the passing.
If we want a ball player, there's the likes of Jorginho, Allan or Julian Weigl, who will sit deeper and spread play, much like Carrick did.
If we want a more mobile all-rounder, there's Sergej Milinkovic-Savic and Leon Goretzka who are capable of getting about, winning the ball and contributing to attacks.
We could also look at Fabinho, who is a good ball winner and passer, and can also cover right-back for us.
We'll need to replace Carrick, the question is whether or not we go like-for-like and get in another deep-lying ball player, or if we get a different kind of player in. There's also question marks over Fellaini renewing his contract, so we could look at bringing in a second player if required. Personally, i'd get a more mobile player than Carrick or Fellaini, allowing Matic to have to do less running, and also switch to 4-3-3 more often, which also helps that. Matic can sit at the base of the three and win the ball and initiate attacks, Pogba can play the more attacking role, driving the ball forward with cover in behind, and the third role would be the player that can do a bit of everything.
