Johnson had enjoyed an excellent pre-season, improving his running and adding some weight to his frame, before his knee gave out when he planted his foot and changed direction last Saturday night against Gold Coast.
The 21-year-old will now head home to Melbourne to decide whether to undergo the LARS procedure and potentially save his season or have a traditional reconstruction.
Johnson hard to replace: skipper
"Obviously it's been a pretty hectic 24-48 hours since it happened," Johnson told the Swans' website.
"I've given it some thought, but I think just getting out of Sydney and getting down to Melbourne and getting my mind off it will give me a chance to make the right decision for myself and the football club.
"Mal (teammate Nick Malceski) and the doctor (Nathan Gibbs) have been through LARS a couple of times and (former Swan) Campbell Heath last year as well.
"I've had a chat to them and now I'm just weighing up my options and I'll give it the next week to make a decision."
It has been a tough situation for the youngster, who played 45 games over his first two years on the Swans' list, including an important role in last year's premiership victory.
"It's obviously pretty flattening," he said.
"I've had a pretty good pre-season and to come to two weeks out from round one to doing my ACL is really disappointing for me and I think the coaches are pretty disappointed too.
"So it's disappointment all round I guess.
"The scans have confirmed I've torn the ACL completely and a bit of damage around the knee, which was expected."
The Swans do have some depth in the backline, with Tony Armstrong, Dane Rampe and Harry Cunningham impressing during the pre-season.
Lewis Roberts-Thomson can also comfortably slot into defence, while Nick Smith (knee) and Marty Mattner (hip) should return from injury over the coming fortnight.
But none of them possess the unique skill set that Johnson brings to the team.
"He's that good height, he's fit, he's fast, so you can't replace him," co-captain Jarrad McVeigh admitted.
"But we've got guys that will certainly step up and will get their opportunity this week and the weeks to come to cement a spot in the team.
"We need those guys to do it.
"(Smith and Mattner) will have to come in now. With AJ going down they'll have to play this week and we'll put out our best side this week and get ready for round one.
"They'll probably play reserves as well, but they have to be ready."
James Dampney is a reporter for AFL.com.au. Follow him on Twitter: @AFL_JD
