Roberto Firmino left Old Trafford on crutches after picking up an injury in Liverpool's goalless draw at Manchester United.
Firmino was brought off shortly after the half-hour mark having rolled over awkwardly on his ankle while chasing the ball.
The 27-year-old had briefly attempted to play on but quickly decided he could not continue and was replaced by Daniel Sturridge.
Firmino was later seen making his way to the Liverpool team bus on crutches and is now a doubt for Wednesday's home meeting with Watford.
Jürgen Klopp, the Liverpool manager, had no information to give on the extent of the damage to Firmino's ankle in his post-match press conference.
Klopp nevertheless expressed his hope that Firmino would be able to recover from the problem swiftly.
"It was something with the ankle," Klopp confirmed. "Bobby said first he could carry on then showed it couldn’t work.
"But I don't know. It’s Bobby so [his recovery] will be quicker than all the others. First we need to know exactly what it is."
Firmino was one of four players to leave the Old Trafford pitch injured in a bizarre first half on Sunday.
United trio Juan Mata, Ander Herrera and Jesse Lingard all departed early with suspected hamstring injuries, while Marcus Rashford played on with an ankle problem.
