The striker missed defeat at Newcastle and faces six weeks sidelined.
O'Neil said: "He is affected by a thigh injury.
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"It will take approximately six weeks, which is obviously a disaster.
"He will probably be back around the same time as Matheus Cunha, maybe Cunha is a bit ahead.
"Statistically, we have stayed in the Premier League in terms of injuries, so we have been competitive, but it is hard to maintain that."