Éder Militão is available for selection after completing his recovery from his ACL injury, Real Madrid coach Carlo Ancelotti confirmed on Saturday.
Ancelotti also said the defender has a "30%" chance of facing Manchester City in their Champions League quarterfinal first leg.
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Brazil international Militão, 26, suffered an ACL tear against Athletic Club in LaLiga on the opening day of the season, but will return to the matchday squad for Sunday's league game against the same opponents at the Bernabéu ahead of the Champions League clash on April 9.
Madrid have had to deal with a string of long-term injuries this season, with both goalkeeper Thibaut Courtois and another defender, David Alaba, also suffering ACL injuries.
"Militão returns tomorrow, which is the most important news," Ancelotti said in a news conference on Saturday.
"He's available, and he's trained. Obviously he isn't 100%, because he needs to play football, but he's OK after the injury... That's very good news for us."
The absentees in defence have seen Antonio Rüdiger partnered by Nacho Fernandez or midfielder Aurélien Tchouaméni at centre-back, although Madrid have maintained an impressive record, topping LaLiga with the fewest goals conceded in the division.
"We'll have to watch [Militão], game by game," Ancelotti said.
"I could give him minutes [against Athletic], and we have time until the 9th to improve his condition. His knee is fine, he's just lacking football, getting used to playing on a big pitch again, and with his teammates."
Madrid host City at the Bernabéu on April 9 before the return leg eight days later in a repeat of last season's semifinal, which City won 5-1 on aggregate.
"It could be [Militão with Rüdiger against City], it could not," Ancelotti said.
"I wouldn't say it's 50%, it could be 70% no and 30% yes. But Militão is a very important player. It helps us in a lot of aspects of play, with aerial balls, with duels.
"He isn't ruled out completely. If he doesn't play, there aren't many options, Nacho or Tchouaméni will play, that's where the doubt is."
Vinícius Júnior will miss Madrid's game against Athletic Club through suspension.
The forward featured for Brazil in their 3-3 friendly draw with Spain at the Bernabéu on Tuesday, after giving an emotional news conference ahead of the game discussing the racist abuse he has suffered from opposition fans.
Ancelotti said he wasn't concerned that the issue could force Vinícius out of LaLiga.
"There's one thing that worries me a lot about Vinícius, and it's that he can't play tomorrow," Ancelotti said.
"That's what worries me."