Aston Villa skipper Martin Laursen is set to miss the remainder of the season with the knee injury which has sidelined him for the past three months.
The central defender has not played since the 2-1 home success over West Brom in early January and he suffered a setback during the club's warm weather training break in Dubai last month and looks certain to miss the final six games of the campaign.
O'Neill said: "Martin is struggling. There's not much more to say. He'll be struggling to play again before the end of the season."
Villa have sorely missed the presence of Laursen at the back and have conceded 24 goals in their last 10 matches.
Striker Gabriel Agbonlahor is improving after suffering a hamstring injury against Everton at the weekend although he remains doubtful for Saturday's home meeting with West Ham.
Villa are still awaiting the results of a scan on the England player.
O'Neill said: "Gabby came off the pitch on Sunday with a hamstring niggle. It seems to be getting better. But we are awaiting the results of a scan.
"At the back of the doctor's mind is a concern that he may have strained it. That would make him doubtful for this weekend's game.
"He certainly seems to be responding - no doubt about that - but he still remains a doubt for the West Ham match."
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