Arsenal boss Mikel Arteta admits captain Martin Odegaard's ankle is "significant".
The midfielder suffered the setback while with Norway last week.
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Ahead of Thursday's Champions League tie with Atalanta, Artera said: "The scan showed some damage around the ligaments in the ankle so we are going to miss him.
"It's something quite significant so we are going to lose him for a while. Hopefully not months.
"He's our captain," added Arteta. "He's been one of the biggest and best players in our team.
"Our identity is very linked to how he plays and behaves. It's a big test for the team to see how able we are to show a different face."