Jarrod Bowen will miss West Ham's Europa League quarter-final first leg against Bayer Leverkusen on Thursday due to a knee injury.
The forward, who has scored 19 goals this season, was injured in Saturday's game against Wolves and could miss the return game on April 18.
Hammers manager David Moyes said: “Jarod is not with us. He twisted his knee and has some damage.”
“Unfortunately, we cannot reveal when he will return.”
Bowen was sent off in the 54th minute. Victory at Wolves He has been a central player for West Ham this season as well.
Moyes said: “He is an England international and we know how important he is to our team so he will definitely be absent, but we have good players who can come in. “I want to think positively,'' he added.
West Ham midfielder Edson Alvarez is suspended, while midfielder Kalvin Phillips and goalkeeper Alphonse Arreola will also miss the game against West Ham due to injury. Bundesliga leader Leverkusen.
“We think this is a great opportunity,” Moyes said. “We've had a good run in Europe and played against a lot of good teams, but we're also playing against new teams.
“Hopefully we can rise to the challenge and do well. Both teams have 11 players so it's an even game to start with.
“We know that Leverkusen are a very good team, but we believe in our abilities.”
Leverkusen lead the Bundesliga with 16 points with six games remaining and are unbeaten in 41 games this season.
West Ham defender Kurt Zouma said: “This could be our toughest challenge yet. They haven't lost for a long time and we know how well they have played this season.”
“But we are still confident. We know the qualities we have and we showed it in the final match against Freiburg.
“We always play to win. We know we may be missing some key players, but we have a good team and everyone is going to give 100%.”
Leverkusen manager Xabi Alonso, who was rumored to be in line to replace Jurgen Klopp as Liverpool manager, has recently confirmed he will take over. Staying at a German Club.
“We know what West Ham are capable of. They have experience in Europe. They were Conference League champions last season,” said Alonso.
“We want to go further and from tomorrow and next week in London we have to play our game. We are ready.”