Ronaldo injury is 'not serious', says Ancelotti
Ronaldo came off at half-time of Real's 1-1 draw at home to Atletico Madrid in the Supercopa de Espana first leg after being largely anonymous on Tuesday.
While further tests are required, Ancelotti said the 29-year-old Portuguese attacker should be OK.
"He felt something and was uncomfortable. We'll evaluate it tomorrow," Ancelotti said.
"Apparently, it was not serious."
James Rodriguez, who replaced Ronaldo, put the hosts in control of the tie with his 81st-minute goal.
But Raul Garcia levelled for Atletico, who are well-placed heading into Friday's second leg at the Vicente Calderon.
Ancelotti believes a similar performance should be enough for Real to win the trophy.
"Atletico do have an advantage for the second leg since now we have to score at the Vicente Calderon," the Italian said.
"If we play like we did tonight we will win the Super Cup.
"The team played well. We didn't have a huge amount of pace in the first half but we improved in the second half.
"We controlled the game pressing up front. Our midfielders were really good."
Ancelotti, who said he had decided on his first-choice goalkeeper for the season, rued the mistake made by his team for Atletico's equaliser.
Koke's corner was flicked in by Garcia, who was unmarked just yards from goal and in front of Iker Casillas.
"We made a mistake on a dead-ball situation," he said.
"We did not cover the front post well and Iker had no chance to do anything in that situation because he had a player in front of him."