Jose Mourinho's assistant has hailed the holding midfielder's importance after he put in a fine performance to help Real Madrid beat Valencia 2-0 on Tuesday
The Germany international was one of Madrid's key performers in the first leg of the quarter-final encounter, and Jose Mourinho's No.2 feels the midfielder has been in impressive form since his return from injury.
"Sami has been in impressive form since his injury. He is a player who may go under the radar a bit, but we know how hard he works," Karanka said at a press conference.
"It's nice to see how he plays, how he get into positions up front and how he picks up balls at the back. He is an important player for us.
"Xabi Alonso and Sami are two players who have been playing together for some time and work hard in those positions. What counts is the form they are in and the confidence they have."
Khedira has made 22 appearances in all competitions for Madrid this season, scoring three goals in the process.