Witsel signed for Zenit for over £30m two years ago, and has become a regular in the Belgian national team nice then, £14m is pie in the sky. He'd be an incredible signing and statement though. Having said that, even if all the players we expect to leave leave, the two central midfield spots are not areas I'd see as a priority - Victor, Cork, JWP, Davis and Reed will do fine for me, with better attacking options in front of them.
In terms of centre backs I can't see us letting Fonte go. Yoshida too will stay and I can easily see us signing a further 2, probably one who can also play left back (Indi/Kongolo?). I'd be delighted with that.