Mikel Arteta says the quadriceps injury Arsenal's Bukayo Saka picked up in the 2-2 Champions League draw against Bayern Munich "does not look good at all".
The 25-year-old winger went down in the 10th minute at Allianz Arena - attempting to play a pass with nobody around him - and had to be replaced by Eberechi Eze, who went on to score the opening goal.
"They are a bit concerned. A player who has to come off with a really sharp pain is never good news but we have to wait and see," Arteta told TNT Sports.
Saka has had fitness issues in recent months and his game time was managed at the end of last season, as well as with England at the World Cup, because of an Achilles injury he picked up in March.
He missed the March international break and seven games for the Gunners, but returned to help them lift the Premier League title for the first time in 22 years.
Saka started the Champions League final defeat by Paris St-Germain but was substituted after 54 minutes, and only started three of England's eight games at the World Cup - scoring a hat-trick in the 6-4 third-place win over France.
The forward also had surgery on a hamstring injury he picked up in December 2024, which kept him out for 101 days in the 2024-25 season.
He made 49 appearances as Arsenal won the title last term, despite a stop-start season - as well as the Achilles issue, he pulled out of the warm-up against Leeds with a hip injury in January and a hamstring problem kept him out for about four weeks.
But he remains vital to the Gunners, whose win percentage is notably more impressive with him in the team.
Despite the loss of Saka early on in the game, Arsenal still produced a good performance and could have won the match late on after Declan Rice's late equaliser.
"The only negative is seeing one of their star players, Saka, having to pull up with an injury early in the first half," said Champions League Match of the Day pundit Nedum Onuoha.
"He was lying on the floor with his hand up and he knew he was in a bit of trouble, so now the concern will be how long he is out for.
"It's not necessarily something that means the team will struggle heavily, but there is a reason why he is regarded as one of the best players in this Arsenal team and is seen as being important for them."
After Saka's withdrawal Noni Madueke, who had started on the left wing, switched to the right and looked bright, while Eze scored the opening goal from the left after coming off the bench.
"This is one of the reasons their squad has been built the way it has been and I think Madueke and Eze did well on the wings today after Saka came off," added Onuoha.
"In terms of options, at this moment in time for Arsenal, it is not going to be an issue - they will miss the star power that he has, they still have plenty of other wide players who can step up and do the job while they wait for him to come back."
Arsenal's style of play has been criticised this season but, even before Saka's withdrawal, the Gunners played on the front foot and enjoyed more possession than their German opponents.
Losing Saka, who has scored four goals in 11 appearances in all competitions, will be tough to deal with.
But the Gunners have invested heavily in their squad over the past two summer transfer windows and have options.
Madueke can play on both wings, while summer signing Christos Tzolis - although currently out with a hamstring injury - has started the season well.
The Gunners also have 16-year-old Max Dowman as an attacking option, and the youngster has already shown that he can have an impact for the senior side after scoring in the Premier League last season.
Arteta, who was asked what the rest of the forward line will need to do in Saka's absence, said he was pleased with what he saw against Bayern.
"I think Ebs [Eze] had a great game. Not only the ball, but the actions. His overall performance, I think his commitment without the ball as well," he said.
"And when the other ones come in, I think they really contributed to this. First of all, the energy.
"And then to bring different tools that the team needed in that moment to get something out of the game. So, very pleased again."