The California welterweight lost a five-round decision to champion Georges St-Pierre on March 16 at Montreal's Bell Centre. He has since complained that Quebec athletic commission (Regie des Alcools des Courses et des Jeux) mishandled its weigh-in and drug testing responsibilities.
And Diaz has asked for a rematch or for St-Pierre to "vacate the belt in favour of those prepared to fight at welterweight."
The UFC offered a generic statement in response.
"We are aware of Mr. Diaz's complaint to the Quebec Athletic Commission," the UFC said in the statement. "We adhere to the policies set forth by respective athletic commissions worldwide."
The UFC provided the statement in lieu of making of vice-president Michael Mersch available.
Mersch has been involved in the controversy because he is the one seen on a UFC 158 weigh-in backstage video relaying supposed Quebec Commission guidelines allowing the main event fighters at extra hour to make weight if needed.
Mersch also passes on the news to the Diaz camp that the Quebec commission does not take fractions into account, so 170.9 pounds would be considered 170.
Traditionally fighters are allowed an extra pound at weigh-ins. So a welterweight can weigh in at 171 pounds, one about the limit. But that extra pound is not allowed in the case of championship bouts.
If fighters do not make weight, they are given an extra hour to do so. If they fail, the fight can go ahead but the overweight fighter owes his opponent 20 per cent of his purse.
Diaz was announced at 169 pounds at the weigh-in while St-Pierre was 170 — the welterweight limit.
The Diaz camp suggests the champion's exact weight may never be known and alleges the Quebec commission "deliberately relaxed the rule in this case and, by its own admission, allowed their home-town fighter to 'make weight' even if he weighed more than the contracted weight."
The Quebec commission confirms that it does not take decimals into account in weigh-ins, but says that rule has been in place for some time and was not there to help St-Pierre.
UFC vice-president Marc Ratner was not at the Montreal fight but said in an email that "my understanding was that both fighters weighed 170 or less" at UFC 158."
St-Pierre's manager did not respond to several emails.