He said: "Theo has improved tremendously as a footballer. He is completely different now in his link-up play with other players.
"His movement is sharper. He is less nervous, more relaxed. In the last six months he has taken a huge step forward.
"He has improved tremendously and he is now a regular player in our team. That has played a big part for him.
"I think Barcelona are the best club side I've ever come up against. I don't deny they are one of the best ever.
"But we want to show how good we are. We are determined. We want to win this competition.
"We face the super-favourites and the only way to do it is to knock them out."