The Ivory Coast international, who picked up the injury during the 2-1 win over Hull on October 19, missed last week's win at Aston Villa and has suffered a setback which has ruled him out of Sunday's home clash with Tottenham.
"We had a little bit of a setback and we are a bit worried with the knock he picked up against Hull," said Martinez.
"I am a bit concerned with his knee because he is someone who gets through anything.
"Little niggles will never stop him from playing or being available and I have never seen him like this. We need to make sure we get to the root of the problem.
"We are going to assess him again and see because it is a little bit more serious than expected.
"He has over-extended the knee. It is something we need to find out about over the next couple of days."
Martinez has better news about Antolin Alcaraz, who has yet to make his Everton debut after being sidelined by a hernia problem since signing in the summer.
"Antolin is progressing really well but he is not going to be ready until the next international break," added Martinez.