The American Gangster dropped the bombshell allegation on his "Good Guy/Bad Guy" show with Daniel Cormier on Monday ... calling BS on Conor's claim that an injury forced him to back out of his fight against Michael Chandler.

The two hosts were discussing how both Conor and Michael have a history of promoting alcohol products ... and while Chael didn't mention the former champ-champ's name directly, it's clear that's who he was talking about.

"What incredible irony that both sides have these massive interests in alcohol, and one side is in the middle of rehab for substance abuse, including alcohol," Chael said ... before joking about how he "wasn't supposed to talk about that."

DC then claimed he didn't hear what Chael said ... so he dropped even more obvious hints -- even referencing Conor's Proper No. 12 whiskey.

"Have this proper drink," Chael -- who recently boxed Anderson Silva -- added. "I'll see you in a proper while, 'cause I'm in a proper facility right now watching me not have drinks."

Earlier in the show, Chael also said people around Conor have been "sworn to secrecy" about the real reason behind pulling out of the fight ... and he would feel bad to "play along" with the injury claim.

Conor's rep was quick to shut down Chael's accusation though ... adamantly denying he's in rehab.

"The fight was cancelled after an injury he sustained during training," Karen Kessler told us. "He is looking forward to a new date."

There's been a lot of question marks surrounding the canceled fight -- like the extent of Conor's injury and what part of his body is busted.

Michael recently opened up on the development ... pouring his heart out in a pretty sad video on Monday.

Conor's social media accounts have been active over the past few days ... but that doesn't really prove anything, as anyone on his team could be behind the posts.