Kevin De Bruyne suffered an injury scare as League Cup holders Manchester City moved into the quarter-finals with a 2-0 win against Fulham.
There was concern for De Bruyne after the City midfielder limped off with what appeared to be a knee injury in the closing minutes on Thursday.
De Bruyne, 27, only recently returned from another knee problem that forced him to miss two months this season, limiting him to five appearances to date.
City manager Pep Guardiola will hope his influential Belgian star is able to make a quick recovery as they head into a busy period featuring crucial Champions League fixtures and the Manchester derby.
"He's being checked by the doctor. Right now I don't know if it's nothing or something serious," Guardiola said.
Until the injury, Guardiola felt it was the most complete performance of De Bruyne's comeback, but now he must fret over his fitness again.
"I think today Kevin is back. The Kevin we know," he said. "Today he was involved in offence and defence. Hopefully what happened in the last minute is not serious."
De Bruyne's injury overshadowed a memorable evening for youngster Brahim Diaz.
Diaz was making only his third appearance of the season and the promising Spanish striker seized his chance with a brace at the Etihad Stadium.
The 19-year-old's goals ensured City advance to face Leicester or Southampton in the last eight as they bid to retain the trophy.
Diaz has been linked with a move to Real Madrid among other top European sides, but Guardiola confirmed City are keen to hold onto their hot prospect.
"He knows the desire we have and what we want for all the young players," Guardiola said.
"They know how we want to protect them and to try to help them understand the way we want to play.
"We want him. In the end it won't depend on us, it is them. It is the agent, him, his family, they will decide."
With the English champions having won at Tottenham in the Premier League on Monday, Guardiola rested many of his stars among 10 changes.
Diaz was one of those who came in, while De Bruyne, Phil Foden and Gabriel Jesus also featured against Premier League strugglers Fulham.
Prior to his injury, De Bruyne had been dominant and he quickly tested the Fulham defence with a run and shot.
Foden brought Fulham goalkeeper Sergio Rico into action with a powerful strike.
But it was his fellow teenager Diaz who opened the scoring with his first City goal in the 19th minute.
A short corner found its way to Vincent Kompany at the far post and Diaz ran on to his header down to hit a powerful shot which deflected past the goalkeeper off Aleksander Mitrovic.
Danilo nearly added City's second when the defender weaved into the penalty area before unleashing a searing effort which struck the outside of a post.
As half-time loomed, De Bruyne sent a wicked cross in from the right but Foden's header went just the wrong side of the far post.
City were straight back on the front foot to start the second half, with Jesus heading wide before De Bruyne's curling shot went just the wrong side of the post.
They had to wait until the 65th minute for the second goal to come as Diaz netted again.
Leroy Sane slipped the ball through to Jesus, but when his shot across goal was touched on to the woodwork by Rico, it fell for Diaz, who took a touch before lashing home.
Fulham never threatened to stop City keeping a fifth successive clean-sheet in all competitions.
A comfortable evening for the Premier League leaders finished on a low note five minutes from full-time when De Bruyne went down in pain after falling awkwardly in a clash with Timothy Fosu-Mensah.
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