Aston Villa have been dealt an injury blow with the news Nigel Reo-Coker is facing up to three months on the sidelines.
The midfielder has suffered a stress fracture to his ankle in training, with scans revealing the full extent of the injury.
Reo-Coker has proved a valuable squad player this season and O'Neill concedes his absence will be felt by the UEFA Champions League chasers.
"Nigel will be out for some considerable time - it's turned out to be a stress fracture - so he might miss 12 weeks," he told the club's official website.
"That's a major blow to us."
Villa's injury woes have been exacerbated by Stephen Warnock's shin problem.
The in-form left-back has been struggling of late and O'Neill concedes he is concerned while he waits to learn of the seriousness of the injury.
"News on Stephen is not fantastic either but we'll see."