Bukayo Saka is set to undergo a scan on a "muscular" injury after limping off during Arsenal's 2-0 victory over Newcastle on Saturday.
Saka gave Arsenal the lead after 56 minutes but was forced off eight minutes later, after going down and pointing to his left thigh.
Arsenal boss Mikel Arteta provided limited details on Saka during his post-match press conference, but the 20-year-old's participation in Thursday's clash with Manchester United now appears uncertain.
"It was an action where he said that he felt something," Arteta said.
"He wasn't comfortable to continue and we'll have to assess him tomorrow and probably scan him and see if there's an injury there."
Saka was replaced by Gabriel Martinelli, who sealed the victory with Arsenal's second goal just 93 seconds after coming off the bench.
Arteta has used Saka in all 13 of Arsenal's Premier League games this season, but the England winger could now be a doubt for Thursday's visit to Old Trafford, with a trip to Everton following four days later, live on Sky Sports.
The comfortable 2-0 victory means Arteta's team are unbeaten in eight games at the Emirates in all competitions, winning seven of those. It's a run of results that has lifted the club to fifth in the Premier League.
"We go game by game," Arteta said when asked about Arsenal's top-four hopes.
"After the defeat at Anfield, obviously you see things in a different perspective maybe, but we've been on a good run and we have to carry on going and let's see where it takes us, but now let's analyse this game and start to prepare the game against Man United as quick as possible because we don't have many days."
Howe: Big call went against us
Former Bournemouth boss Eddie Howe was in the Newcastle dugout for the first time having finally tested negative after previously contracting Covid-19. He could not shift the losing mentality within his group though with Newcastle still yet to win a league game this season.
There was not much in the way of big chances falling their way but Callum Wilson had a penalty appeal waved away by Stuart Attwell just before Martinelli doubled the lead. Howe felt it was a penalty.
Howe said: "I thought it was a good performance, especially the start. We were good for the first half an hour. We came under pressure. The game was decided in the incident with Callum Wilson and the penalty - a tough call against us - then they scored in the aftermath.
"I don't know why Callum would go down - he was one on one, a great chance. Games swing on those moments. It feels like those big calls have gone against us.
"Great save [from Aaron Ramsdale to deny Jonjo Shelvey]. I thought it was in. The pleasing thing today is how we started and finished the game. We created chances. Ramsdale has played well. We weren't clinical but at least we were creating chances.
"The players have been very responsive. They have given everything in the two games. I can't ask more than their best."