It is a big blow to the Premier League club who have been in impressive form since the turn of the year, including a 4-2 win at Chelsea in which Cunha scored a hat-trick.
He suffered the injury in last weekend's game at Brentford.
"It's a fairly significant injury to his hamstring. There's no timeline on it. I've spoken to Matheus a lot and he just wants to keep pushing to get back as soon as he can," O'Neil said.
"He's started working on it already, so we're just going to monitor it week by week, day by day.
"It's obviously a big blow to us, to the team and Matheus because of the work he's been doing -- the form he was in was exceptional. These things happen, every team suffers injuries."
Wolves are in 11th place in the Premier League standings and face Tottenham Hotspur away on Saturday.
