Aston Villa left-back Stephen Warnock is battling to overcome a shin injury in time for Wednesday's vital Premier League clash with visiting Arsenal.
The former Blackburn Rovers player had to be substituted early in the second half of Saturday's home FA Cup fourth-round win over Brighton.
O'Neill will want Warnock fit for the Gunners clash as Villa look to step up their bid for a Champions League spot.
He said: "Warnock has been sore for a little while. He wanted to go again (against Brighton) but he's played a lot of games for us.
"Ideally I wouldn't have minded him not playing, but at this minute Nicky Shorey is on loan at Forest and, even if he came back, would have been suspended at the weekend.
"Stephen is a great lad who would play when a lot of other players wouldn't risk it as much so we'll have a look at things.
"He will try and rest up for a day or two and see how he is for Wednesday."
O'Neill added: "Stephen doesn't remember getting hit on the shin but our doctor seems to think he probably has at one stage or another and it's really irking him."