Klaassen signed for Werder in a €15m deal this week and scored the winner in their 1-0 friendly victory over Arminia Bielefeld. The midfielder admits former Ajax teammate Niklas Moisander was a big influence.
He said, "He has only told positive stories about the club. I, of course, also follow the Bundesliga. And I know that Werder is a traditional club.
"The ambitions of the club and the way of playing under this trainer (Florian Kohfeldt) have been important.
"After the talk with Kohfeldt in Manchester, I was convinced. He told me how he wants to play and what my role is. After that it was clear to me that I wanted to go to Werder."