Sunderland have been hit by an injury blow to Carlos Cuellar, with the defender facing a spell on the sidelines due to a hamstring problem.
The Spaniard was replaced by Titus Bramble at half-time during the 1-1 draw with Liverpool at the Stadium of Light on Saturday and has been receiving treatment for the problem since.
The former Aston Villa and Rangers player is expected to miss the clash with West Ham on Saturday, and could also be absent for the forthcoming fixtures against MK Dons and Wigan.
O'Neill said: "Carlos Cuellar has had a hamstring problem so he will be very, very doubtful, not just for this game, but perhaps for the next couple of matches
"So from that viewpoint it is a bit disconcerting, but these things happen and you just have to get on with it."
Adam Johnson did not feature against the Reds last weekend after suffering a minor thigh injury which forced him out of the recent England squad, and O'Neill admits he is still unsure as to whether he will feature against the Hammers.
The Black Cats boss added: "We were hoping that he would have made the fixture against Liverpool last weekend, but that wasn't the case.
"He is progressing and it's too early to say about this weekend."