The former Whites defender, who now serves as voice of the club during Jose Mourinho's quiet phase, has spoken about the club's latest signing from the German top flight
The Turkish midfielder announced on Monday that he will be leaving the newly-crowned Bundesliga champions for Jose Mourinho's side this summer for around €10 million on a six-year deal.
Sahin joined the German club as an 11-year-old, making his first-team debut in 2005 at the tender age of 16. Madrid's new signing is still only 22, something Karanka is fully aware of.
"Sahin has shown his quality in Germany and we await big things from him. We will be patient though, as he is 23 [this year]," Karanka told Goal.com Spain.
"The signing is very positive for us. The club is already working on next season and he is a player that has had a sensational season. We will see how he adapts to Madrid, but we are happy because we have high hopes for him."
Sahin will be joining other ex-Bundesliga stars Mesut Ozil and Sami Khedira who arrived in Madrid last summer and both proved to be astute purchases.
Karanka was keen to highlight the club's policy of adding young yet talented players to their ranks with Sahin matching the criteria.
"The transfer of Sahin is due to the line which the club took a couple of years ago - young but experienced players who proved their quality in important league," he added.
Sahin has enjoyed an excellent campaign with Dortmund, who have already clinched the German title, scoring six Bundesliga goals and providing eight assists.