The nontitle welterweight limit is 171 pounds and Chimaev weighed in at 178.5 pounds. Diaz was 171 pounds.
Chimaev, 28, is one of the UFC's biggest rising stars and was a -1000 favorite, per Caesars Sportsbook.
When Chimaev weighed in Friday morning, he shrugged his shoulders and showed no contrition for the massive miss. Later, he posted a meme joking about missing weight on Twitter.