“It was a bizarre and unique situation,” Matchroom promoter Hearn said in a news conference.
“I was ringside when I was told to come backstage ASAP. Gomez fell ill in the changing room, he was bent over in pain.
“The doctors took a urine sample, and he had an infection, he was not fit to fight. They completely ruled him out.
“I have never known it an hour before the fight. It’s a weird one.”
Gomez’s illness came as a surprise as he took part in the usual pre-fight build-up – a news conference and weigh-in – without any obvious issue.
At 15.41 BST on Saturday, he posted an official promotional image to X with a message for fans to “watch my fight tonight”.
The 29-year-old local hope had been looking to challenge for British and Commonwealth titles against Bellotti.
The Manchester-born fighter has won 21 of 22 fights in his career, including six knockouts.
The fight would have formed part of the first boxing event held at the new arena in Manchester, before the headline bout between Catterall and Prograis.