Gareth Bale is out of Real Madrid's Champions League clash with FC Copenhagen on Wednesday after missing training with his teammates on Tuesday.
The Welshman had instead taken part in an individual session in the gym, but he has been left out of coach Carlo Ancelotti's 21-man squad for the game against the Danish champions.
"Gareth Bale has not trained with the group due to muscular problems in his left thigh," said a statement on the club's website.
"The diagnosis is dependent upon tests which will be carried out tomorrow."
The 24-year-old also missed league games against Getafe and Elche last week due to a left thigh problem.
However, he returned to make his home debut as a second-half substitute in the 1-0 defeat to Atletico Madrid on Saturday.
That defeat left Real already five points behind Atletico and Barcelona, who have both made 100 percent starts to La Liga after seven games.
And Ancelotti said he expects a reaction from his players upon their return to Champions League duty.
"I'm less happy after the game against Atletico because it wasn't good, losing a derby is never a good thing," he told a press conference on Tuesday.
"We need to react quickly, both in terms of our performance and results. I have confidence in the players that they will respond well. It is a question of spirit and attitude."
Defender Sergio Ramos also backed Ancelotti to turn things around and insisted that Real will compete for all of La Liga, Champions League and Copa del Rey this season.
"Now is the time when we need to be most united," he said.
"It is true that we are playing badly and that people are not feeling comfortable, but we will get there because it is our obligation and we have a great team. Let nobody be in doubt, we will be fighting for the three titles.
"The team is more united than ever, with a great atmosphere, a coach with whom we are more than happy and the results will come."
Madrid's struggles to adapt to Ancelotti's possession based game after three years where they were more dangerous on the counter-attack under Jose Mourinho were particularly exposed by Atletico.
However, Ancelotti believes it will just take some time for his new troops to adjust to a new style of play.
"We can do more. We are not comfortable in possession at the minute and we have lost some of our defensive balance. Right now we are not winning the ball back well enough, but it is still very early."
However, in Europe, things have started extremely well for Madrid with their 6-1 hammering of Galatasaray two weeks ago allied to Juventus' 1-1 draw with Copenhagen handing the Spanish giants an early advantage in Group B.
And Ancelotti says the point Copenhagen took from his countrymen should serve as a warning to his side not to take the Danish champions lightly despite their own domestic struggles.
"They have had the same system and coach for a while. We know they haven't had a great start in the league, but they drew against Juventus so we have to be alert."
Iker Casillas is set to make his first competitive appearance at the Bernabeu for nine months as Ancelotti intends to play the Spanish captain in the Champions League and Copa del Rey whilst Diego Lopez maintains the number one jersey in La Liga.
Ancelotti may also recall Marcelo and Raphael Varane to the defence as they have both returned to full fitness.