Gareth Bale limped out of Real Madrid's latest Champions League outing, putting him in doubt for their La Liga clash against Barcelona.
The Wales forward appeared to injure his right ankle 58 minutes into their Group F match during a play involving Sporting defender Sebastian Coates.
"Bale suffered an injury, but he needs a thorough examination tomorrow [Wednesday] to see the extent of it," Madrid manager Zinadine Zidane said. "It is too soon to say how long he will be out for or the size of the problem."
Bale tried to play on after treatment, but signaled that he could not continue after about a minute.
"We will see, so I don't know yet if he will be for the Clasico," Zidane said.
The first meeting between the two La Liga sides this season is at Camp Nou on December 3.
Bale had been in fine touch in the first half in Lisbon as Real Madrid took a 1-0 lead through Raphael Varane. After he went off they lost their advantage to a penalty, but a late winner from Karim Benzema secured the 2-1 result.
If Real beat Borussia Dortmund in two weeks at the Santiago Bernabeu, they will top the Champions League group, but that fixture is just three days after their El Clasico encounter.
Bale could miss both matches.
Real have a frantic schedule with the Club World Cup starting in Japan on December 8 and running through December 18.
They are four points clear of Barcelona atop the La Liga table with 30 points from 12 games this season.