Middlesbrough striker Mustapha Carayol will undergo surgery next month after scans revealed he has torn his anterior cruciate ligament.
The 25-year-old was injured during Saturday's Sky Bet Championship draw at Bournemouth and now faces months on the sidelines.
A club spokesman said: "Following scans and consultations in London, it has been confirmed that Muzzy Carayol has torn his anterior cruciate ligament.
"He will be operated on by world-renowned English specialist Andy Williams in two weeks' time once the swelling has gone down."
Carayol had only just returned to the team after head coach Aitor Karanka had criticised his attitude.
He had started the last three games and scored in the 1-1 draw with Nottingham Forest on March 11.