Saka was forced off during Saturday night's 5-1 win at Crystal Palace.
He went off on 24 minutes and was later seen on crutches.
"He felt something in his hamstring, he couldn't continue," said Arteta. "He will have to be assessed. Pretty worried about that one.
"It's difficult. They have tested him but it's very difficult to say how bad it is.
"I'm not sure if it's exactly the same one (from earlier this season), it's in the same leg.
"I don't know how big this is going to be but he's already had two."
Raheem Sterling has also collected an injury.
The manager continued: "We don't know for how long. 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."