The fullback was forced off on 70 minutes in victory over Bournemouth.
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Slot said: "The situation was that he said to me, 'You have to take me off' and he sat on the floor and we took him off.
"He felt something, I can't tell you exactly what and how bad it is of course because we are one hour after the game, but it is never a good sign if a player is asking for a substitution.
"This is not a fact but I would be surprised if he plays on Thursday, but let's hope he is back with us as soon as possible."