Liverpool midfielder Georginio Wijnaldum needs a scan on an ankle injury and is doubtful for Saturday's Premier League trip to West Ham.
The 26-year-old received a knock to his right foot in the first half of their 3-0 Champions League win over Maribor.
"At half-time I asked immediately and the ankle was already swollen," said manager Jurgen Klopp. "It is probably not a good sign."
Philippe Coutinho is also set to miss Saturday's game with a muscle problem.
Coutinho, 25, has missed the past two games, against Huddersfield and Maribor, with the adductor problem and Klopp does not think that the Brazilian will be fit to play.
Injury problems mount for Reds
Dutchman Wijnaldum has missed two games this season - the 1-1 league draw against Burnley on 16 September and a 4-1 defeat by Tottenham on 22 October.
His collision with a visiting player looked innocuous at first, but the former Newcastle midfielder was unable to continue and was replaced by Jordan Henderson.
Mohamed Salah, Emre Can and Daniel Sturridge scored to maintain Liverpool's unbeaten start in Group E.
Klopp said: "We saw a kick in the game and did not think it was too serious. Then it was swollen. We're not sure if he twisted it or not. For sure, we have to make a scan."
Wijnaldum will join an injury list which contains Coutinho, while forward Sadio Mane is not expected to return until mid-November and defenders Dejan Lovren and Joe Gomez missed the Europe game with knocks.
Long-term absentees Adam Lallana and Nathaniel Clyne are yet to feature for the club this season.