Gabriel Martinelli will not feature against Southampton on Thursday after Arsenal head coach Mikel Arteta confirmed the striker will be out for "months" with a knee injury.
Martinelli sustained a knock during training, a minor cartilage injury in his left knee on Sunday, and the Brazilian is set to undergo more tests in the forthcoming days.
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In a press conference on Tuesday, Arteta said: "We had some really bad news in training again yesterday. The last action he [Martinelli] got hit by another player and he has damaged his knee.
"The doctors are assessing the extent of the injury but it is not looking good at all.
When asked how long the forward will be unavailable for, Arteta said: "We don't know but it is looking [like] months."
Arsenal have also confirmed Bernd Leno has been ruled out for up to six weeks with a moderate ligament strain, defender Pablo Mari will not play for the rest of the campaign and will return in two or three months, while Sokratis Papastathopoulos and Granit Xhaka have returned to full training.
During Arsenal's 2-1 defeat to Brighton, Leno was stretchered off with a knee injury before half-time at the Amex Stadium on Saturday.
Brighton forward Neal Maupay went up for an aerial challenge with the Germany goalkeeper and forced Leno onto the floor. Leno then reacted angrily towards Maupay as he left the field with the assistance of Arsenal's medical team.
Concerning Leno's setback, Arteta said: "We thought it was going to a more significant injury, we don't know the full extent of the injury still but we do know that the cruciate ligaments are not damaged - it was the biggest fear at the start.
The Gunners head coach said his injury has given a chance for reserve goalkeeper Emiliano Martinez to deputise in place of Leno: "He [Martinez] has been very patient for a lot of years, waiting on this opportunity.
"He needs to take it, enjoy it and make the most out of it. Help the team when we really need it, that's why he is here."
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Mari, meanwhile, has been seeing a specialist after he suffered a severe ankle injury which saw him forced off 24 minutes into Arsenal's 3-0 defeat to Manchester City, in the opening fixture of the Premier League restart.
Xhaka is available for selection after he sustained a sprained ankle during that game, while Lucas Torreira remains sidelined.
The Gunners are suffering from a lack of defensive cover with David Luiz also suspended for another two matches, after he was shown a straight red card at the Etihad Stadium on June 17.
Luiz was Mari's replacement against City.
Shkodran Mustafi and Rob Holding both played the full match against Brighton, with the pair being Mikel Arteta's only centre-backs currently available for selection.
Watch Southampton vs Arsenal live on Sky Sports Football and Main Event on Thursday from 5:30pm, kick-off 6pm.