"We had a little bit of a setback and we are a bit worried," said Martinez.
"Little niggles never stop him from playing, I've never seen him like this.
"We need to make sure we get to the root of the problem."
The Spaniard added that Kone will have to undergo further tests before the full extent of the problem is known.
"We are going to assess him again and see because it is a little bit more serious than expected," Martinez said.
"He has over-extended the knee. It is something we need to find out about over the next couple of days.
"We have to do a couple more tests to see how the knee is reacting."
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