Arsenal manager Mikel Arteta says he is "pretty worried" about Bukayo Saka after the England winger went off injured against Crystal Palace.
Saka suffered a hamstring injury during Saturday's 5-1 win at Crystal Palace and had to be replaced in the 24th minute.
The 23-year-old was using crutches as he left Selhurst Park.
"He felt something in his hamstring, he couldn't continue," said Arteta. "He will have to be assessed. [I'm] pretty worried about that one."
Asked how long Saka might be out, Arteta replied: "It's difficult. They have tested him but it's very difficult to say how bad it is."
Arteta added that fellow winger Raheem Sterling has also picked up an injury, ruling him out for Saturday's game.
"We don't know for how long," he said. "To lose two players in 24 hours is not good.
"Unfortunately, with the amount of games that we are playing and with this schedule, it doesn't surprise me. But the team will react to that."
Saka has only missed two games for Arsenal this season, after he suffered a hamstring injury while playing for England in October.
He also went off injured against Chelsea in November, ruling him out for England during the last international break, but was able to start in the Gunners' next game against Nottingham Forest on 23 November.
Asked if this is Saka's third hamstring injury, Arteta said: "I'm not sure if it's exactly the same one, it's in the same leg.
"I don't know how big this is going to be but he's already had two."