Pep Guardiola says Kevin De Bruyne will be checked over by Manchester City doctors after the Belgian injured his knee in the 2-0 Carabao Cup victory over Fulham on Thursday night.
De Bruyne was substituted late on after Fulham's Timothy Fosu-Mensah fell backwards onto his knee.
The 27-year-old appeared to be in some discomfort and walked gingerly from the pitch. Upon being substituted he seemed to indicate to Guardiola that he had suffered an impact injury, before heading down the tunnel.
De Bruyne missed 10 matches with a knee ligament injury earlier this season, though that was his right knee and on Thursday he appears to have injured his left.
After the match Guardiola told reporters that the club's medical staff would carry out further tests.
"We do not know if it is nothing or something serious," he said.
De Bruyne was making only his second start of the season against the Cottagers and was in fine for at the Etihad Stadium, having returned to the starting line-up against Shakhtar Donetsk last week but not risked from the beginning on a poor pitch at Wembley on Monday.
"I think today Kevin is back, no? It’s the Kevin we know," Guardiola said.
"He made a huge effort in Donetsk against Shakhtar. He played a few minutes in a difficult situation against Spurs.
"It was a difficult situation because of the pitch and the conditions and tough physicality but today again he was involved in the offensive and defensive side, in the pace, in the set pieces, the passes and assists.
"Hopefully what happened in the last minutes is not serious."