Liverpool manager Jurgen Klopp says Jordan Henderson's injury "doesn't look good" after the midfielder hurt his knee in Thursday's 1-1 draw at Borussia Dortmund.
The England international was replaced at half-time by Joe Allen in the Europa League quarter-final first leg and will have a scan on Friday.
"It is not a small thing," Reds boss Klopp said.
England boss Roy Hodgson is due to announce his Euro 2016 squad on 12 May.
Henderson, 25, was out for three months earlier this season after damaging his heel against Bournemouth in August.
His latest injury means he could miss the return leg at Anfield on 14 April.
'People thought we would lose 2, 3 or 4-0'
Dortmund beat Tottenham 3-0 at home in the last 16 of the Europa League but were given a much tougher test by Liverpool.
Divock Origi gave the Reds the lead in the first half with a low finish but was denied a second by Dortmund keeper Roman Weidenfeller before Mats Hummels equalised just after the break with a header following a corner.
"I am satisfied with a lot of parts of the game," said former Dortmund boss Klopp. "Dortmund are still a very strong team but people thought a lot about our possible performance so they are perhaps a little surprised now.
"I'm pretty sure a lot of people thought we would lose 2, 3 or 4-0.
Liverpool have progressed from 11 of their past 12 European ties in which they have drawn the first leg away from home.
Klopp, though, does not believe Liverpool have the edge, adding: "I think we saw the game is still 50-50 and Dortmund don't care too much about where they are playing or where they score a goal.
"An away goal is really good, 100% better than a 0-0."
Dortmund have scored in each of their previous six away games, and boss Thomas Tuchel said: "We are able to score away goals as well.
"We are not frightened and we are not too disappointed. We went to a lot of places and scored and won. It's half-time and the players are confident."
'A special place to come to'
The fixture marked a first return to Dortmund for Klopp since he left the Bundesliga side at the end of last season.
After taking charge in 2008, Klopp won two league titles with the German club and led them to the final of the 2013 Champions League.
"There was applause which I took for me and I made a small gesture in return," he said. "The people here are very special and I felt good coming back."