Alex Oxlade-Chamberlain was forced off through injury in the first half of Liverpool's Champions League semi-final first leg against Roma at Anfield.
The England international was hurt 15 minutes into the game after making a sliding tackle on former Manchester City defender Aleksandar Kolarov.
Oxlade-Chamberlain appeared to hold his knee initially and, after receiving treatment on the touchline, was carried away on a stretcher.
Georginio Wijnaldum - who missed out on a place in the starting XI, with manager Jurgen Klopp opting for James Milner instead - replaced his stricken team-mate in the 18th minute.
Speaking post-match at Anfield, Klopp told BT Sport: “Oxlade, it’s probably a really bad injury. If we can say that already before the scan, that’s never good news so that means for us it’s a massive blow, of course. We have still a few very important games to go and the squad doesn’t get bigger in the moment, so we obviously need to be creative in the next few games.”
The injury is not just a setback for Liverpool, who travel to Italy next week for the second leg of the tie against Roma at the Stadio Olimpico, but also potentially England. National team manager Gareth Southgate will be concerned over Oxlade-Chamberlain's condition ahead of the World Cup in Russia.