Arsenal forward Gabriel Jesus suffered a suspected ACL injury during the FA Cup match against Manchester United on Sunday.
Jesus started the cup tie against United but was forced off after just 41 minutes and replaced by Raheem Sterling.
"We know more now. It is not looking good at all," said Arsenal boss Mikel Arteta ahead of Wednesday's North London Derby.
"We need to review with one more specialist and will have better information this afternoon.
"I don't want to confirm anything until we have the final report this afternoon. We were very worried after the game and we are very worried today."
The 27-year-old has seen his Arsenal career blighted by injury since arriving from Manchester City in 2022 and is now set for an extended period on the sidelines alongside forwards Bukayo Saka and Ethan Nwaneri.
The Brazil international had scored six goals across his last seven appearances for Arsenal, who could now be forced into action in the January transfer window to acquire support for Kai Havertz in the centre-forward position having sold striker Eddie Nketiah to Crystal Palace in the summer.
"We are actively looking in the market to improve the squad," added Arteta. "We would be naive not to do that. There is an opportunity to improve the squad.
"We are looking and trying. Let's see what we are able to do. When you recruit a player there are three parties. We also have our limitations, we can do what we can do."
When pushed for an answer on whether or not a forward was the number one priority, he added: "In any position.
"We have other issues as well. A player can bring things to a team in different ways. That is what we are trying to do."
' Nick Wright:
Arsenal's main recruits in the off-season were Mikel Merino and Riccardo Calafiori, with Raheem Sterling arriving later on loan. All three have struggled to make a positive impact, with Merino and Calafiori only making 12 starts each, and Sterling just five.
Injuries have not helped. All three have encountered issues, with Calafiori suffering a new setback which ruled him out on Sunday. But there are question marks hanging over them irrespective of injuries. Were they really what Arsenal needed to go to the next level?
Merino, for example, has brought aerial strength and duel-winning ability to Arsenal's midfield when it was probably in greater need of creativity and goal threat, especially having sold Emile Smith Rowe to Fulham and allowed Fabio Vieira to join Porto on loan.
The club hoped Sterling might bring those qualities in attack when they took the chance to sign him on loan. But it is damning that Arteta has rarely turned to him, even when he has been fit. Halfway into his spell, he has played a paltry 146 Premier League minutes.
He will likely be needed now, with Jesus having joined the injury list. But, like Merino and Calafiori, he has work to do to show he can give Arsenal and Arteta what they need.
Arteta talked up his "love" of his players in the wake of Sunday's defeat but it is becoming increasingly obvious the only solutions to their attacking issues lie in the transfer market. The problem is that the January window is far tougher to navigate than the summer one.
Arteta says the abuse Havertz suffered following the defeat to Manchester United "cannot happen" and called for "perspective" to the Arsenal striker's struggle in front of goal.
"It is incredible, honestly," said Arteta. "We really have to do something about it because accepting it has terrible consequences. It is something we have to eradicate in the game. It is dependent on action.
"On 27 December we win 1-0. Havertz scores a goal and the whole stadium is singing his song. That was just 20 days ago, so where is the perspective? We are all responsible for the narrative and we cannot look somewhere else. It is a really serious matter.
"The line has to be drawn. It cannot happen. That is what we should aim for."
Arsenal reported the social media abuse to Signify, a tech company that investigates online abuse directed at players. The club also reported the accounts to Instagram.
News understands Arsenal will take the strongest possible action against the culprits.
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