Kevin De Bruyne survived a scathing response from Manchester City to the quarter-finals with a 2-0 win over Fulham on Thursday.
There was anxiety about De Bruyne after City midfielder missed the knee after knee ligaments.
De Bruyne, 27, recently returned from another knee problem that forced him to miss two seasons this season, having played five games so far.
City boss Pep Guardiola hopes the influential Belgian will be able to recover next season, playing for important Champions League and Manchester derby.
"The doctor is investigating, and now it's not clear if it's nothing or something serious," said Guardiola.
Until he was injured, Guardiola felt he was the best player in the career of De Bruyne, but now he must go back to his fitness.
"I think today Kevin is returning, Kevin we know," he said. "Today he was involved in crime and defense, we hope that what has happened in the last few minutes is not serious."
De Bruyne injured Brahim Diaz at night
Diaz only made it a third season this season, and the Spaniard is hopeful he will be able to find the Etihad Stadium.
The 19-year-old has confirmed that City will be facing Leicester or Southampton in their last eight matches as they seek to win a trophy.
Diaz has been linked with a move to Real Madrid, but Guardiola has confirmed City are keen to keep their hopes alive.
"He knows what we want and what we want all the young players," said Guardiola.
"They know how we want to protect them and try to help them understand how we want to play.
"We want him, and he will not rely on the end, he is their agent, he, his family, will go."
With the Premier League champions winning the Premier League this week, Guardiola has rested 10 different players.
Diaz was one of those who came in, while De Bruyne, Phil Foden and Gabriel Jesus were also against Fulham.
Prior to his injury, De Bruyne was an independent and quickly tested Fulham's defense and frustrated the game.
Foden returned to full-back Fulham's Sergio Rico on the attack.
But he was Diaz's young player who opened his first goal for City in the 19th minute.
Judge Vincent Kompany scored, Diaz's header from the edge of his shot and the ball goalkeeper Aleksander Mitrovic.
Danilo has almost joined the club's second-tier City defender after the defender was penalized in the penalty area before the ball struck his effort off the line.
Subsequently, De Bruyne gave the hosts the lead, but Foden only headed in the head with just a bad shot from the far post.
City went straight back to the first leg to start the second half, with Jesus leading the ball ahead of De Bruyne to his left hamstring.
They have to wait until 65 minutes of the second goal for Diaz again.
Leroy Sane passed through the ball to Jesus, but when he hit Rico in the win, Diaz fell to the ground before falling home.
Fulham never threatens to quit City to retain its fifth consecutive win in all competitions.
The Premier League title of the season has completed a full five minutes when De Bruyne gets sick after suddenly striking Timothy Fosu-Mensah.