Zinedine Zidane admitted he was worried about Eden Hazard's ankle injury suffered in Real Madrid's 2-2 draw with Paris Saint-Germain.
A Karim Benzema double looked to have sealed an impressive victory for the Blancos, but goals from Kylian Mbappe and Pablo Sarabia earned the visitors an unlikely draw late on.
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Hazard was substituted in the second half at Santiago Bernabeu with a twisted ankle, after going down under a challenge from Belgian compatriot Thomas Meunier.
Zidane, whose team are now guaranteed to finish second in Group A, is concerned about the injury blow.
"Yes, it's a worry," the Madrid coach told Movistar.
At his news conference, Zidane added: "It is more than a simple blow, but I hope it is less. I can't say right now.
"They have now gone to do tests and we will see."
While unhappy with the final result, Zidane said the performance from Madrid was pleasing.
"It has been a great match for the team. We keep growing. You have to be happy with the game," he said.
"Then with the result, you cannot be happy. But you have to be happy with what we did for 80 or 85 minutes."
Madrid are now unbeaten in seven matches across all competitions, including five wins, and look well placed to fight for major silverware once again.
Zidane's men are second in La Liga on goal difference, and can now look forward to featuring in the Champions League knockout stages when February rolls around.
Next up for the Blancos is a trip to Deportivo Alaves on Saturday, before they host Espanyol at the Bernabeu seven days later.
After their final Group A Champions League fixture away to Club Brugges on December 11, Madrid will then look forward to crucial domestic fixtures against Valencia and Barcelona.