The former Chelsea winger has hardly played since his arrival due to injury, but Mijatovic argues: "People can have that feeling because the player has not played as much as we want.
"He arrived with an injury and when it was fixed, he hurt himself with Holland. There, yes, he was injured again.
"Now he is struggling, but he will respond and be decisive in next the four or five seasons. He is only 23 years old and has much quality. We just need him to relax a little bit, because it has not been mentally easy for him.
"It is too soon to say that we have been mistaken, we need more time. Robben will be very useful."