Koeman said: “He can play against them because he is registered to us for the whole season.
“People said they may try and bring him to England in January, but the deal was for 12 months, so we would have tried to keep him.
“He is a fantastic defender and played so well tonight. In the really big games, the best players show themselves. He enjoyed the level this game was played at. Sometimes, in the Dutch league he loses concentration more easily because the level’s not as high. But he’s one of the best central defenders and I’m amazed that he is still playing with us.”