How many times have we conceded late into a half since Sherwood took over too? That can probably only be put down to a lack of concentration from them all. Clark and Okore would be first choice centre pairing as they could form a partnership for years. Bacuna can get better defensively (though he's already decent) and is far too beneficial to our attacking play to not use (4 assists when he's started already). Left back we might need a new signing there. As far as cover goes, Hutton, either Richardson or Bennett and Baker/Senderos. If Vlaar goes, we might need another due to injury track records. We might need a signing there unless there's someone coming through the ranks.
I think that Vlaar's the weak link. If Senderos is fully fit, I'd prefer to see him alongside Okore.