Zinedine Zidane is unsure whether Gareth Bale will be fit for El Clasico in 11 days' time after the Real Madrid forward suffered an injury to his right ankle during Tuesday's 2-1 Champions League win over Sporting CP.
Bale was caught by a challenge from Sporting centre-back Sebastian Coates early in the second half and, despite attempting to play on after receiving treatment, he departed with 58 minutes on the clock.
LaLiga leaders Madrid host Sporting Gijon this weekend and will hope to have their four-point advantage at the summit still intact when they travel to Camp Nou to face bitter rivals Barcelona on December 3.
"Bale has an ankle problem, but he needs a thorough exam tomorrow to see the extent of it," said Zidane after the Sporting match, where Karim Benzema's header three minutes from time secured their passage into the knockout stages of the Champions League.
"It's too soon to say how long he will be out or the size of the problem. We will see, so we don't know if he will be fit for El Clasico."
Madrid had to work hard for the points against Sporting, who levelled in the 80th minute through Adrien Silva's penalty despite Joao Periera having been sent off for punching Mateo Kovacic in the stomach.
Zidane's men remain two points behind Borussia Dortmund, but can usurp the Group F leaders by beating them at the Santiago Bernabeu in the matchweek six.
"Maybe Sporting deserve more points, maybe so," said Zidane of his team's opponents, who will now attempt to fend off Legia Warsaw and secure passage into the Europa League.
"We now have a final against Dortmund. We knew it was going to be a tough game here.
"We have to rest well because the next game comes very soon."
Madrid are now unbeaten in 30 matches across all competitions and Zidane added: "We want to continue what we are doing, knowing that in games there are times when you have bad moments.
"These 30 games are a motivation to move forward with our work. Will it continue as well? We do not know because physically it is not easy for everyone. "