An injury to Kevin de Bruyne took the shine off a convincing win for Manchester City as the Carabao Cup holders eased past Fulham to advance to the quarter-finals.
Spanish teenager Brahim Diaz scored in each half - his first senior goals - to set up a last-eight tie against Leicester City or Southampton.
But there was concern before the end for De Bruyne, who grimaced as he limped off three minutes from time.
The 27-year-old Belgium midfielder was making only his second start of the season on Thursday after suffering a ligament injury to his right knee in August, the BBC reports.
"He is being checked by the doctors," said City manager Pep Guardiola. "We don't know if it's nothing or if it's something serious."
De Bruyne had put in a typically impressive and energetic performance but went off after going to ground clutching his left knee, and disappeared straight down the tunnel for treatment.
The Premier League leaders made 10 changes from the team that beat Tottenham on Monday but still outclassed a full-strength Fulham side, managing 27 shots to five from the visitors.
Diaz broke the deadlock after 18 minutes, meeting Vincent Kompany's knock-down with a first-time shot that took a huge deflection and span past Sergio Rico.
He added his second on 65 minutes after Rico had tipped a Gabriel Jesus shot onto the post, with the 19-year-old following up to hammer his shot into the roof of the net.
City will find out their quarter-final opponents on 27 November, when Southampton travel to Leicester.
Their fourth-round tie was originally meant to be played on Tuesday but was postponed after Saturday's helicopter crash at King Power Stadium that killed Foxes owner Vichai Srivaddhanaprabha and four other people.