Kaoru Mitoma did not train with his Japan teammates on Monday ahead of their Asian Cup group decider against Indonesia, with defender Ko Itakura admitting it was "a worry."
The Brighton winger has not played since hurting his ankle in a 1-1 draw with Crystal Palace on Dec. 21.
Japan coach Hajime Moriyasu included him in his squad for the Asian Cup but the player has not been included in the matchday squad for their two games in Qatar so far.
The 26-year-old Mitoma worked out alone at the team hotel while the rest of the players trained ahead of Wednesday's game against Indonesia.
Japan has not yet qualified for the last 16 and needs at least a draw to guarantee its place.
Borussia Monchengladbach's Itakura said Mitoma was on the players' minds.
"It's a worry," he told reporters. "I hope he recovers from his injury quickly."
Japan beat Vietnam 4-2 in their opening game but was stunned 2-1 by Iraq and now has no chance of topping their group.
The side potentially faces a last-16 clash with archrivals South Korea if they both qualify for the knockout rounds.
Japan began the tournament with forward Takefusa Kubo and defender Takehiro Tomiyasu also working their way back from injury.
Kubo has appeared in both games so far while Tomiyasu came on at half-time against Iraq.
Real Sociedad's Kubo said Mitoma's absence was "not a problem."
"Mitoma isn't here at the moment, but football is not a game that you play on your own," he said.
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