How many central midfielders do we need now?(not many) How many will we need in the future when, hopefully, we are involved in more matches).
Locks for next season:
Gueye
Schneiderlin
Barkley
Davies
This leaves
Barry
McCarthy
Besic
I think Barry will see out his contract (1 more year), but I think/hope he will be relegated to a more elder statesman role and will only play in cup matches and as injury cover.
Besic could be sold, but I think he plays how Koeman likes and with his injuries you won't get much for him. He is worth the risk of putting on the squad and hoping he has a healthy year.
McCarthy has shown this season (Arsenal, Liverpool, end of the city match) that he does provide something to the team and I do think Koeman likes what he brings to the table and would like to keep him. However, I don't think he would be a starter and at his age I don't think hed be satisfied playing in a subs role. You also would be able to get the most money for him.
I think McCarthy is the most likely to be sold and he makes the most sense.While a summer deal may be the when this happens, since we are out of the cups and the emergence of Davies, I think we can afford to let him go at the end of the winter window if teams can pay the appropriate price (20M+). Newcastle does make a good amount of sense. They do pull in a massive amount of revenue and seem favorites to go up. So I don't think McCarthy would mind doing 4 months in the championship and being closer to home (I assume he still thinks of Scotland as home), rather then going to a PL this window that looks more likely to be going down.