After posing with the iconic performer, Diaz then sent out an image of himself holding what has been reported as a rolled up joint.
There is no footage of Diaz smoking the cigar-like stick.
It's less than two weeks since the United States Anti-Doping Agency confirmed it had opened an investigation into Diaz for smoking a vape pen following his loss to McGregor.
Diaz even confirmed in a post-fight interview that cannabidiol oil was the substance inside the vaping device.
"It's CBD," Diaz said.
"It helps with the healing process and inflammation, stuff like that. So you want to get these for before and after the fights, training. It'll make your life a better place."
CBD has been used to treat epilepsy and schizophrenia. Studies have shown that it has fewer side effects than THC and does not cause any psychoactivity.
Cannabidiol is one of 113 active cannabinoids in cannabis. Cannabinoids are banned in-competition by the World Anti-Doping Agency (WADA) Code over a certain threshold and USADA uses WADA's prohibited substance list.
Even though the fight was over when Diaz was vaping CBD, the timing still constitutes as in-competition if it's six hours after a fight. Diaz's media duties were within that time frame.
"I can confirm that USADA is aware of the situation and is currently gathering information in order to determine the next appropriate steps," USADA spokesman Ryan Madden told MMA Fighting in a statement.
Cannabis is considered a "specified substance" by the UFC anti-doping policy, meaning it's not an anabolic or hormonal agent but is still prohibited in certain cases, like in-competition. Any fighter violating the policy for a specified substance can be banned for up to one year.
Depending on circumstances, USADA can give cannabis users in-competition anywhere from a public-warning sanction to a one-year suspension.
A drug test must have more than 150 ng/ml of marijuana metabolites to be considered positive. In this case, Diaz's fight-night drug tests have not returned yet, so no one knows if he tested positive or not. Diaz was tested before he got to the press conference.
Diaz has never failed a drug test in his MMA career.
