I don't think we need another CB as long as Vermaelen's injury is a one off. Djourou has kept up his form long enough to be our second choice. Kosicelny has been fine as third choice and with room to improve and I think he will grow because that's one thing he's been pretty consistent at. Squillaci has been our best fourth choice defender in years and he's finishing the season strongly - ahead of Kosicelny atm.
Generally speaking we've chopped and changed our goalkeeper and center backs and still joint-second in the least number of goals conceded which I'm pretty pleased with. And partnerships do need time to form while players settle down, I think we'll only know where we stand if we can finally go with a majority of Szczesny, Djourou, Vermaelen, Kosicelny playing together for a season to see whether they can work together and be a unit.
Put it this way, I feel that our existing defenders and goalkeepers will improve. I do not feel Denilson, Diaby, Bendtner, Chamakh and Vela will ever make the step up. Therefore, I feel we need a striker, winger and midfielder before we start reeling in defenders.