Arsenal defender Takehiro Tomiyasu is reportedly at risk of missing the rest of the season through injury.
Tomiyasu, 24, limped off in Thursday's Europa League round of 16 second leg against Sporting Lisbon, which ended with Arsenal being eliminated from the competition on penalties.
He was later seen wearing a brace and using crutches and subsequently missed the 4-1 win over Crystal Palace in the Premier League on Sunday.
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Tomiyasu pulled out of the Japan squad for the current international break and Arsenal boss Mikel Arteta admits there are serious concerns over the injury.
"We've had one assessment, they need to have another assessment tomorrow, but it's not looking good, that's for sure," Arteta said on Sunday.
"Looking at Tomi, he's not someone who exaggerates things. He was really concerned straight away, and we are concerned."
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Injury expert Ben Dinnery wrote on Twitter that Tomiyasu 'may not play again this season', while prominent leak account Team News and Ticks claimed the Japanese defender required surgery and would miss the rest of the campaign.
Tomiyasu injury 'not looking good'
Meanwhile, Arsenal reporter Charles Watts says there are fears over Tomiyasu's availability for the Premier League title run-in.
"I'm afraid Tomiyasu, again, it's not looking good. We know that," said Watts on his YouTube channel.
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"There's not been a kind of official announcement yet. They're still going through the final sort of checks. But it looks bad.
"All the comments coming from Arsenal are suggesting it's bad, but they're waiting to get the final results for all the tests that he's gone through."
Arsenal also have concerns over William Saliba, who was also forced against Sporting and missed the Palace win with a back injury.
It has since been reported in the French media that Saliba could face several weeks on the sidelines.
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"We hope Willy is going to be better news but again we have to wait to understand a better picture," Arteta added.