Romero (12-2) could not make the 185-pound limit at Saturday's weigh-in. He initially came in at 188.5 pounds and had two hours to drop the remaining pounds. When he stepped back on the scale, he weighed in at 187.7 pounds.
The two will remain on the card. If Rockhold wins, he'll be the interim champ, according to UFC president Dana White.
"Romero took this fight on short notice when [Robert] Whittaker fell out, and he didn't get a full camp," White said. "I appreciate him stepping up and taking the fight."
Rockhold (16-3) is coming off a second-round TKO win against David Branch in September.
Romero hopes to rebound from a decision loss to Whittaker last July. He previously won eight consecutive bouts.
Heavyweight veteran Mark Hunt and rising contender Curtis Blaydes are also on Sunday's card.