Carlo Ancelotti said he has "never seen three cruciate ligament injuries in four months" after David Alaba joined Thibaut Courtois and Éder Militão on Real Madrid's long-term injury list, with the coach saying the club could now consider a move for a defender in January.

Centre-back Alaba was substituted in the 35th minute of Madrid's 4-1 LaLiga win over Villarreal on Sunday after his left knee appeared to buckle as he challenged for the ball in the centre circle at the Santiago Bernabeu.

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"After tests carried out on our player David Alaba, he's been diagnosed with an anterior cruciate ligament tear in his left knee," Madrid confirmed in a statement. "The player will undergo surgery in the next few days."

"I haven't spoken to [Alaba]," Ancelotti said in his postmatch news conference. "It's a shame, obviously. We're very sad to lose another player. I've never seen three cruciate ligament injuries in four months. It's incredible. But there's nothing we can do.

"What the team has to do is what it's done with all the injuries we've had, which is to hang on. We've done more than people thought we would. With all our problems, this team is battling, fighting, with extraordinary commitment.

"It's the only way we can replace all the important players who've suffered injuries. We'll see in the next few days if we can do something [in January]."

Ferland Mendy was also substituted at half-time with muscular discomfort, while Dani Carvajal, Eduardo Camavinga, Arda Güler and Vinícius Júnior are all currently on the sideline with Militão and Courtois. The coach said Carvajal could play Alavés on Thursday.

"I think he'll train with the team on Tuesday," Ancelotti said of Carvajal. "I didn't want to take any risks with Mendy. He felt something in his abductor, so to avoid it getting any worse I preferred to take him off."

The Italian said that midfielder Aurélien Tchouaméni -- who returned from a broken toe as a substitute against Villarreal -- could be drafted into defence, and said Militão could return from his ACL tear before the end of the season.

"Tchouaméni is the first option we have right now," Ancelotti said. "It isn't where we see him, but as an emergency -- and now it's an emergency -- he can play there... [Academy player] Marvel has trained with us, but he's injured now. After Christmas we'll see what we can do."

Teammate Lucas Vázquez said Alaba was upset after his injury.

"It's a difficult, long injury," Vázquez said. "We're having some bad luck. [Alaba] is a top professional, we're all sad."

Jude Bellingham opened the scoring on Sunday with his 13th goal in 15 LaLiga games before being substituted in the second half.

"He was important, he played very well, fighting," Ancelotti said. "I took him off to avoid problems. Bearing in mind that he's had his shoulder issue [after dislocating it against Rayo Vallecano] when I can save him some minutes, I will."

Madrid's win put them top of LaLiga on 42 points, one clear of Girona, who play Alavés on Monday.