Kaoru Mitoma will miss Brighton's last two matches of the Premier League season and could be a doubt for the World Cup.
The Japan winger suffered a hamstring injury during Albion's 3-0 win over Wolves last weekend.
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"He will miss the next two games," Brighton boss Fabian Hürzeler told reporters ahead of Saturday's trip to Leeds.
"Regarding the World Cup, I'm not sure yet. We'll be in touch with the Japan federation.
"It is a big blow. We have a big squad and we have showed this season we can replace important players. We see it as a chance to stick together.
"We really have to be patient with this injury, we cannot give a clear schedule.
"He will miss our two games. That is sad for Kaoru. He was in good form and had a big impact on our performances."
Japan open their Word Cup campaign against the Netherlands on 14 June in Arlington, Texas.
They will also play Tunisia on June 20 in Monterrey, Mexico, then return to Texas to play Sweden on June 25. The top two in Group F advance to the round of 32 and the third-place team also could join them.
Mitoma, 28, has nine goals in 31 appearances for Japan.
"I hope it's a minor injury. I was told by staff, 'It doesn't seem minor,'" Japan coach Hajime Moriyasu was reported telling Japanese media Sunday about Mitoma.
PA and The Associated Press contributed to this report.
