Chimaev tipped the scales at 178.5 pounds, 7.5 pounds over the welterweight limit, with his team saying a "medical issue" prevented him from cutting weight effectively.
Diaz tipped the scales at 171 pounds, but Chimaev's miss means the main event is unlikely to take place.
Chimaev may be replaced by American Tony Ferguson (38) or Chinese fighter Li Jingliang (34), who both weighed in within the limit for their welterweight co-main event.
Thursday's news conference was abandoned after trouble backstage, with fighters Chimaev and American Kevin Holland getting into an altercation followed by a row between Chimaev and the Diaz entourage.
With both sides of that scuffle well represented in the auditorium, UFC president Dana White closed down the news conference quickly, rather than risk an outbreak of violence.
"You don't have to be a rocket scientist to figure out where this whole press conference was headed, and there were so many people with all the camps that once that started that would have been bad," White told reporters in a backstage scrum.
The Las Vegas Metropolitan Police greeted people arriving for Friday's official weigh-ins at the UFC's Apex building.
