Gary Cahill is set to be out for four months following an operation to repair damaged ankle ligaments.
'Gary had an operation yesterday and he'll be out for about four months at least,' O'Leary told the club's official website.
'So that makes us very brittle centre half wise.
'The bottom line is that we have two fit centre halves at the club now in Ridgewell and Mellberg so we cannot afford to have any injuries in that position now, until the Christmas window.
'He's damaged ankle ligaments and other things. The surgeon didn't like what he saw but he's repaired it and it will be a long job.'