The Brazilian has scored nine league goals for Wolves this season, including a hat-trick against Chelsea earlier this month.

"It's obviously a big blow to us, to the team and Matheus because of the work he's been doing," O'Neil said.

"The form he was in was exceptional."

The Wolves boss told the club's website that there is "no timeline" on Cunha's recovery.

"He's started working on it already, so we're just going to monitor it week by week, day by day, see how far he can come and how quickly we can get him back," O'Neil added.

Hwang Hee-chan, Wolves' top scorer in the league this season, has returned to the club after being at the Asian Cup with South Korea, but a calf injury saw him miss the loss against Brentford.

"Channy [Hwang] was gone for over a month with the Asian Cup and we managed to keep ploughing on and we need to do the same without Matheus," O'Neill said.

Wolves, who are 11th in the Premier League, travel to Tottenham on Saturday.