Coleman was helped from the field at halftime on Friday at the Gabba after his left leg clashed with Carlton tackler Lachie Fogarty as he kicked.
The Lions had blitzed the Blues early but in the third term had eight-straight goals kicked on them in a 13.8 (86) to 12.13 (85) loss that ended a 14-game winning streak at their Gabba stronghold.
Coleman was sent for scans on Saturday which confirmed the Lions' worst fears of a season-ending injury.
"He'll get a scan tomorrow, but at this stage it looks like an ACL," Lions coach Chris Fagan said in the post-game press conference. — AFL (@AFL) https://t.co/BwVyTjd9eTMarch 8, 2024
"It's really disappointing news for Keidean, especially in the first game after a strong pre-season and an impressive finals campaign last year," Lions football manager Danny Daly said in a statement.
"However, Kiddy (Coleman) is only young and has a very bright future ahead of him with the Brisbane Lions, which we all look forward to.
"Kiddy is an extremely positive and calm person, so I am confident those traits will no doubt help him get through his recovery over the coming months."
Conor McKenna could return from a hamstring injury next week while Will Ashcroft won't be back until the latter half of this season after his own ACL setback in 2023.
But Coleman's ball use and dare out of defence will be hard to replace; the 23-year-old was dominant in the first half of last season's grand final.
"We get disappointed for all the lads with those sorts of injuries, because we know (it's) a long way back," Lions coach Chris Fagan said after Friday night's defeat.
"It'll create an opportunity for somebody else, which will be their good fortune."
Fellow flag contenders Port Adelaide have also suffered an injury blow, with key forward Ollie Lord to miss the start of the season after a training mishap.
Lord landed heavily in a marking contest at training on Saturday which caused high-grade bone bruising on his knee and an impaction fracture in his tibia.
The 22-year-old will consult a specialist in the coming days to determine whether surgery is required.
The Power are yet to put a time frame on Lord's recovery.
