Real Madrid have confirmed that Cristiano Ronaldo will miss out on Saturday's derby clash against Atletico Madrid due to a leg injury.
The Portuguese star scored his side's away goal in their 4-1 UEFA Champions League defeat to Dortmund on Wednesday night and is not being risked this weekend ahead of the return leg in Spain on Tuesday evening.
The Liga champions, who are 13 points off Barcelona with just 18 to play for, will also be short of defensive options, with Marcelo and Alvaro Arbeloa also struggling.
"Cristiano Ronaldo will not be in the squad for tomorrow," Karanka said.
"The other day before the [Dortmund] game he had a little injury, he has not trained today and he won't play tomorrow. We will see how he progresses."
"With the exception of Cristiano Ronaldo, Marcelo and [Alvaro] Arbeloa, the rest are well and tomorrow our best team will go out to win."
Karanka believes that a strong performance against Diego Simeone's side would be the perfect preparation for the meeting with Dortmund, which Real must win by a scoreline of 3-0 or by at least a four-goal margin to progress.
He added: "Next Tuesday will be the second leg, but it is hugely important we do not lose this game because they are our rivals for second place, the rivals of the city.
"So, tomorrow's line-up is going to go out with the idea of winning the game. We are only thinking about tomorrow's game in our squad, not Tuesday.
"They [Atletico] haven’t won this game since a long time. We have to be 120 percent like them. They have had a good run at home this season, almost perfect.
"As I said, we are fighting against them for second place. Besides, it is the derby of the city, they have spent a long time not winning this game and they really want to win so we have to go with the highest intensity."