Cristiano Ronaldo may miss Real Madrid's trip to in-form Sevilla in La Liga on Sunday after the Portuguese winger damaged his right ankle in Wednesday's 3-0 Champions League win over Olympique Marseille.
Real's record signing scored twice against the French club but was felled by defender Souleymane Diawara in winning a second-half penalty and had to be replaced by Gonzalo Higuain.
"Cristiano has some swelling and we'll have to wait and see," coach Manuel Pellegrini told a news conference after the victory at the Bernabeu that put Real top of Group C.
"The state of the ankle shows that he took a hefty knock in the penalty incident," added the Chilean. "It's still early to say whether he can play against Sevilla on Sunday."
Ronaldo, who has five La Liga goals already this season and four in the Champions League, will have tests on Thursday to determine the extent of the injury and has been doing some gym work, Real said on their website.
Sevilla, third last term, have a chance to prove they can cut it with the La Liga big beasts in Sunday's match (1900 GMT), 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 (1800).
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 (2000).
"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."
Promoted Xerez secured their first point in the top flight with a 0-0 draw at Espanyol last weekend but have failed to score a goal in five matches this season and play at home to Malaga in the Andalusia derby on Sunday (1500).