Fact is, buying a CB this window wouldn't be a bad thing (depending on price) anyway.
We've got 3 first team CBs- so we're 1 down on what is usually acceptable.
Gomez is young and has had a number of injuries- so he probably should be rotated anyway, for both reasons
Matip is made of matchsticks and breaks too easy- so its unlikely he will ever be fit enough to be first team regular and he's not getting any younger.
All in all, we need at least One CB anyway and frankly we should be looking for another to replace Matip who appears to be abit of a lost cause.