Ronaldo doubtful for Sevilla showdown
The Portuguese forward scored twice at the Bernabeu, his eighth and ninth goals in seven matches since his record transfer from Manchester United.
He suffered the injury when Marseille defender Souleymane Diawara fouled him and conceded a penalty in the second half.
After undergoing tests on Thursday morning, Ronaldo trained separately from the rest of the squad in the gym, Real said on their website.
Real top the La Liga standings with Barcelona after winning their first five matches of the season.
Sevilla, third last term, have a chance to prove they can cut it with the La Liga big beasts in Sunday's match, according to striker Alvaro Negredo.
After losing 2-0 to Valencia in their opening game, Sevilla have won four on the trot and are the only team who appear capable of mounting a sustained challenge to the dominance of Real and champions Barcelona.
Real and Barca have a perfect 15 points from five matches, with Real top on goal difference, and Sevilla have 12 in third.
Atletico Madrid and Villarreal, fourth and fifth last term, have suffered poor starts and are down in the relegation places on just three and two points respectively.
Pellegrini's expensively assembled Real side have won all their competitive matches so far this season but have been unable to dominate opponents with the possession football practiced by arch rivals Barca.
"The individual quality of the Madrid players means they can beat anyone but we're going to take them on," Negredo said on his club's website.
"If we want to be up there we have to take three points at home," he added. "Of course they are the clear favourites but we are playing at home and that means it will be more even."
Pep Guardiola's Barca side have some injury worries of their own as they prepare to host Almeria on Saturday.
France striker Thierry Henry missed Tuesday's 2-0 win over Dynamo Kiev with a muscle strain and Ukrainian central defender Dmytro Chygrynskiy is out for three weeks after he damaged a knee ligament in last weekend's bruising clash at Malaga.
"The match is coming after we played in the Champions League and sometimes it's hard to focus," midfielder Xavi told the club's website.
"Almeria will have had the whole week to prepare and have been studying us," he added. "It won't be easy."
Atletico president Enrique Cerezo has backed coach Abel Resino despite the club's woeful start and they have a chance for a first victory of the season when they host Real Zaragoza on Saturday.
"I feel like I have what it takes to carry on here and sometimes football depends on your state of mind," Resino said after Atletico's 2-0 defeat at Porto on Wednesday.
"The effort from the team is there and a victory would give us a boost in confidence."