Wolves midfielder Joe Hodge is out two months with a shoulder injury.
Hodge has undergone surgery to correct a nagging problem.
“Hodgey has had surgery on his shoulder," O'Neil said.
"It's been ongoing for a while, he had an issue with it coming out too many times over the past few weeks in training, disrupting him, so we took the opportunity to get that fixed.
"Hopefully that's him in a slightly better place. He'll be missing for a while."
Asked for a date over Hodge's return, O'Neil added: "I can give you a rough guide from my limited knowledge on shoulder surgeries and I would guess somewhere from eight and 12 weeks.
"But he only had it done a day or two ago, so it's still early. It was good to see him this morning and he was in good spirits now it's fixed and he feels he's on the mend. Hopefully we can get him back as soon as possible."