"I'm worried about the whole Borussia Dortmund team," said the Portuguese. "It is no coincidence that they have won the last two league titles in Germany or that they are top of the most difficult group in this season's UEFA Champions League.
"They are a very strong team with very physical, quick players. If they reach the next round they will be candidates to win the trophy.
"It's a tough group, every point you get is an achievement, every team in this group is at risk. If we beat Borussia Dortmund, we will be in a better position."
In the other game in Group D on Tuesday, Manchester City will host Ajax at the Etihad Stadium.