Diaz, 32, reposted an image to Instagram on Tuesday that was originally shared by the UFC's account. The image shows McGregor accompanied with the quote, "I never lose. Either I win or I learn."
Diaz, who fought McGregor twice in 2016, apparently did not agree with the UFC's interpretation of the boxing match, which McGregor lost via TKO in the 10th round.
"He punched himself out the same way he lost in the Ufc [sic] there was no learning goin on.. #overpromotion," Diaz wrote. "Bulls--- get off the nuts this is the s--- I'm talking about. Bruce Lee would've never lost like that."
Diaz talked up McGregor's chances of defeating Mayweather prior to the fight. After Mayweather won, photos appeared of him posing with Mayweather in the undefeated boxer's locker room.
Immediately after his loss to Mayweather, McGregor (21-3), who currently holds the UFC's lightweight championship, admitted he needs to improve his overall cardio -- both in boxing and MMA. The phrase, "Win or Learn" is a staple of his SBG Ireland team and the title of a book authored by his longtime coach, John Kavanagh.
Diaz, who submitted McGregor at UFC 196 before dropping a majority decision at UFC 202, has complained in the past of a discrepancy he sees in the way the UFC promotes McGregor from its other athletes.
He said the UFC pushed the narrative of McGregor's "obsession" with a rematch after their first fight, when, in reality, any fighter who loses wants an immediate rematch.
Diaz (19-11) hasn't fought since the second McGregor fight in August 2016. The UFC expressed interest in booking him to a fight in July, but the two sides were unable to reach a deal.
McGregor, of Dublin, has repeatedly stated their rivalry will eventually be settled in a third fight. The Irish star is expected to fight again in either December or early 2018.