Gareth Bale handed Real Madrid an injury concern as he was substituted after 58 minutes of Tuesday's Champions League Group F match at Sporting CP.
The Wales international has seven goals in 16 appearances for the European champions this season and displayed impressive form once more during the first half in Lisbon, where Raphael Varane gave Madrid a 1-0 lead.
But Bale took a blow to his right ankle in a challenge with Sebastian Coates early in the second period and, despite receiving treatment, his attempts to continue proved unsuccessful.
Madrid are four points clear of Barcelona at the top of LaLiga, with the first Clasico of the season 11 days away on December 3 at Camp Nou.
Zinedine Zidane's side host Sporting Gijon on Saturday.