Before his injury Djourou and Koscielny went on a run of 8.5 games in the league, conceding 2 goals. 1 against Chelsea, and the second a goal that should have never stood against Everton.
I think Djourou and Squillaci have a similarly impressive record.
And despite missing our best defender for the whole season, our second best defender for half a season, our two best goalkeepers for half the season and generally playing an attacking brand of football, we still have the second best defence in the country behind Chelsea.
We really don't need to bring in a centre back. I wouldn't say no, but it's well down the list of priorities behind a winger, striker and competition for Wilshere/Song.