PSV Eindhoven have kept their slim Eredivisie title hopes alive with a 3-1 victory away to AZ Alkmaar to move into second position.
Mesut Oezil's double and Karim Benzema's strike secured the win at the Santiago Bernabeu, but it came at a cost with Marcelo and Luka Modric forced off while Benzema also picked up a knock.
And Karanka has admitted their possible absence for the Champions League semi-final clash with Borussia Dortmund has taken the gloss off the result.
"Karim Benzema's injury doesn't look too bad thankfully but Marcelo's could be," the 39-year-old said.
"He felt a bad pain in his leg, and Luka Modric at first looked to be just a muscle overload but later he has said that he felt pain as well in the muscle.
"We will have to wait for the doctors to assess the players and let us know. We will wait and see what happens but that is the worst thing that happened in this match.
"I do not think goalkeeper (Diego) Lopez has anything serious though. We'll make changes as soon as we evaluate Marcelo's condition, but the important thing was to win.
"(Cristiano) Ronaldo and Oezil played the full 90 minutes with no problem. For Oezil, playing the whole match was important. It was a good chance to gain confidence."
The Spanish champions opted to start with Castilla midfielder Casemiro, and Karanka was left impressed by what he saw from the Brazilian and fellow youngster Nacho.
"The boy did really well and gave everything tonight," he continued.
"Casemiro is a player we know well, one who has played in Brazil's first division and is an Under-20 world champion.
"We want players of personality and experience. Nacho always delivers too, and has shone for Castilla as well."
Real consolidated second in the league with their latest win, six points clear of local rivals Atletico Madrid, who have a game in hand.