Martin Odegaard will be out for the foreseeable future after medical assessments showed that the Arsenal captain has sustained “significant” ankle ligament damage.

Odegaard has undergone numerous scans and has seen a specialist since rolling his ankle in horrible fashion while on international duty with Norway this month.

Norway’s team doctor said after the initial incident that Odegaard could be out for around three weeks, but it now seems clear that the injury is more severe than was first feared.

Odegaard will therefore not be available for Arsenal’s Champions League opener against Atalanta in Bergamo on Thursday night, and will also miss Sunday’s heavyweight showdown with Manchester City in the Premier League.

Mikel Arteta, the Arsenal manager, said he was hopeful that Odegaard will not be out for “months”, but made it clear that the 25-year-old is not expected to be back in action any time soon.

“After everything was scanned, the scan showed he has got some damage, especially on one of the ligaments in the ankle,” said Arteta. “We are going to miss him. I don’t want to [give a timeline] because I am not a doctor but yeah, it is something quite significant so we are going to lose him for a while. Hopefully not months, but let’s see.”

‘Our identity is related to his way of playing’

Asked if he is confident he has the players capable of replacing Odegaard, Arteta added: “Yes, but we have to show that. He is our captain, he is one of our biggest players without a doubt. He has been one of the most consistent players in the last two or three seasons.

“Our identity is very related to his way of playing and his way of behaving. We are going to have to deal with that. That is a great test for the team as well, of how capable we are to show maybe a different face and the fact we have other players and other options that can still be very effective.”

Arsenal were able to cope without Odegaard and the suspended Declan Rice in Sunday’s north London derby against Tottenham Hotspur, but the Norwegian’s continued absence is a significant blow for Arteta, who is also without new signing Mikel Merino due to a serious shoulder injury.

The club’s midfield crisis has been slightly eased by the return to action of Rice, who is set to start against Atalanta.

Odegaard’s injury is also likely to present more opportunities for teenage playmaker Ethan Nwaneri, who is looking to establish himself as a first-team regular this season. The 17-year-old impressed in pre-season and came off the bench against Spurs this weekend, playing the final few minutes as Arsenal held on to their 1-0 lead.

Arsenal have been boosted by the return to training of Italy defender Riccardo Calafiori, who also missed the north London derby with an injury he picked up on international duty.

Fellow left-back Oleksandr Zinchenko did not train before the trip to Italy, however, as he continues his recovery from a calf issue. Defenders Takehiro Tomiyasu and Kieran Tierney are long-term absentees.

Arsenal will come up against former defender Sead Kolasinac in their match against Atalanta. Kolasinac was among the players who had their contracts terminated as part of the squad overhaul following Arteta’s appointment in north London, with the full-back leaving the club in 2022.

“I’m happy to see my old team-mates again, it’s special,” said Kolasinac. “It’s always difficult to play against English teams, they are physically strong.

“They have great players, are solid in defence and very dangerous at set-pieces. We are up against a team with an excellent shape, who can press high while remaining compact. They are a young side who know their strengths, and we know ours.”