The 24-year-old, who is on an 18-month loan at his former club Dortmund from Real Madrid, says that although he thought Porto may progress against Malaga, he knows all about the Spaniards' qualities from his time playing in La Liga.
"We were worried about Porto, but they were knocked out by Malaga,” Sahin told reporters. “I know them from my days at Real Madrid. They played very well against us when I was in Spain. We should definitely not underestimate Malaga."
The first leg will take place at La Rosaleda on April 3, with the return leg at Signal Iduna Park on April 9.
Sahin also gave his opinion on the tie between Galatasaray and Real Madrid, saying the Turkish side is the obvious underdog but have the quality to pull the upset.
“I believe Galatasaray have a slight chance to advance. I think it’s a positive thing to play the first leg away," Sahin said. “A draw without conceding a goal or a draw with lot of goals can open the way for Galatasaray to get a good result in the second leg at home, in front of their own fans."