But both fighters vow ring rust will not be an issue in their December 10 matchup at Los Angeles, which could set the winner on a collision course with unbeaten WBA "super" featherweight champion Carl Frampton of Northern Ireland.
Mexican-American Mares, 29-2 with one draw and 15 knockouts, will be two weeks past his 31st birthday when he meets Cuellar, 28-1 with 21 knockouts and an interim champion or better in the weight class for three years.
The bout had been scheduled for March and then postponed to June after New York boxing officials were concerned about left eye surgery Mares underwent in 2008.
"I've been cleared in every other state since," Mares said. "It was just a red flag for them because I had surgery done in 2008. We're good to go. I have the same risk as any other fighter out there."
Mares says the layoff could be a blessing in disguise, saying ring rust "wears off between one and two rounds."
"You have to see it as a positive. I've been active since March. I've practically done training camp for two fights. You will see a much more mature fighter."
Cuellar, 29, is on an 11-fight win streak over the past five years and shrugs off any notion the layoff will hurt him.
"I feel very good. I've had a great training camp. I'm looking forward to this fight. I have a lot of confidence," he said through a translator.
"I have the size advantage but I cannot rely on that. This is going to be a difficult match. Abner has a lot of talent."
Mares was a world bantamweight champion in 2011 and a world super bantamweight champion in 2012. In 2013, he took the World Boxing Council featherweight title but surrendered the crown in a knockout loss to Mexico's Jhonny Gonzalez that year.
In August of last year, Mares lost a majority decision to Mexico's Leo Santa Cruz for the crown now held by Frampton and has not fought since.
"I gave a great fight against a hell of a fighter," Mares said. "I have a lot left. This is my 'second wind.' It's about your heart and your willingness to be hit. I've showed that's what I have deep inside."
Beating Mares helped launch Santa Cruz (32-1-1 with 18 knockouts) into a New York title bout last July against Frampton (23-0 with 14 knockouts), who took a majority decision but will fight a January rematch in Las Vegas.
On the Cuellar-Mares undercard is unbeaten American Jermall Charlo (24-0 with 18 knockouts) defending his International Boxing Federation junior middleweight crown against undefeated compatriot Julian Williams (22-0 with one draw and 14 knockouts).