Eden Hazard hobbled off during Real Madrid's La Liga game at Levante on Saturday - giving Zinedine Zidane a major injury concern ahead of a crucial week for the club.
The Belgium international, who has only recently returned from a fractured ankle, was clearly in pain as he was replaced by Vinicius Junior in the 67th minute with the score at 0-0.
Hazard could be seen sitting on the visitor's bench receiving treatment to his lower right leg, with Zidane now waiting to discover the full extent of the problem ahead of two huge home fixtures.
Madrid, which went on to lose 1-0 to Levante, hosts Manchester City on Wednesday in the first leg of their last-16 Champions League tie, followed by the visit of title rival Barcelona in league action next Sunday.
Hazard was making just his second outing since requiring surgery on his right ankle after he was hurt against Paris Saint-Germain on November 2.
His debut campaign in the Spanish capital got off to a delayed start following a thigh injury in pre-season, forcing him to wait until September 14 to make his debut following his arrival from Chelsea in June.
-Hazard injury 'doesn't look good', admits Madrid boss Zidane-
Zinedine Zidane admitted Hazard's latest injury "doesn't look good".
"It doesn't look good," Zidane told reporters at a news conference.
"It can become weak where you've had an injury. It is a blow, let's see.
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"It's sore now but we'll see tomorrow when we do more tests."
Madrid drew 2-2 with Celta Vigo at home in its previous outing and it will host Barcelona next Sunday knowing it is now two points behind its great rival in the title race.
Captain Sergio Ramos revealed the level of frustration he was feeling in a social media post.
"We're p***ed off," he wrote on Twitter.
"But we can't allow our heads to drop.
"We have to reset now and tomorrow push harder than ever before for a very important week. We're counting on you."