The 23-year-old was forced to leave the field on a stretcher as the Gunners beat Chelsea 2-0 on Saturday and boss Unai Emery immediately feared the worse.
Arsenal had been waiting for the swelling to go down before scanning the injury, with reports emerging late on Monday night that the injury is a serious one.
It is claimed that Bellerin will need further scans to determine whether their is any damage to the meniscus and other parts of the joint, but he is looking at nine months on the sidelines.
That will rule him out for the rest of the campaign and means he could also miss the start of next season.
The Gunners have yet to make any official announcement, but the injury details and his rehabilitation timeline is expected to be revealed at some point today.