Arsenal manager Mikel Arteta says captain Martin Odegaard has suffered a “quite significant” ankle ligament injury.
The midfielder limped off during Norway’s Nations League win over Austria on Monday, 9 September.
Odegaard had gone down in pain after hurting his left ankle following a foul by Christoph Baumgartner midway through the second half.
He returned to Arsenal for an MRI scan and missed his side’s 1-0 win at Tottenham in the north London derby on Sunday.
“The scan showed some damage around the ligaments in the ankle so we are going to miss him,” said Gunners boss Arteta before his side play Atalanta in the Champions League on Thursday.
“It’s something quite significant so we are going to lose him for a while. Hopefully not months.”
Odegaard’s injury means the 25-year-old will miss the trip to Italy and his second-placed side’s visit to leaders Manchester City in the Premier League on Sunday.
“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.”
Source: BBC