Mate having seen a fair bit of Witsel, Koeman will have earmarked a midfield partner for him whose game compliments each other, thats the way to get the best out of him, he partnered up mainly with Garcia (the old Benfica/City player) at Zenit who played that more defensive role and allowed Witsel to roam a bit more, and it worked a treat, can see a similarly big physical player playing along side him - which we honestly don't have right now.
Honestly do not see McCarthy and Witsel gelling that well together and would bet the house on us bringing another centre mid in with more presence/physicality about them
....I've been making this point about blend and partnerships. The coach will have a team model in his mind and the types of players who not only fit into it but who compliment each other. I might not necessarily be a fan of some names being mentioned but I'm not the one putting the team together. I just hope the manager gets his main targets.