"But that's not only in fuel efficiency, it's in economics," Hazy told Flight International affiliate FlightBlogger at the Singapore air show.
"Maintenance costs will have to come down. When you're dealing in a high-cycle environment maintenance costs become a much more important driver than in a long-haul environment."
He says Airbus and Boeing are exercising caution over a replacement, and adds: "I think both programmes have moved a little to the right."
Hazy says that the A350 and 787 is consuming a lot of the airframers' "internal talent and resources financially. I think the narrowbody replacement will become more of a focus once the 787 is certificated and the A350 is further along in terms of its cycle."
He adds that ILFC has been "advocating" the development of twin-aisle narrowbody aircraft with seven-abreast seating - which the lessor has dubbed "Airbus A360" and "Boeing 797" - but adds that such an aircraft would depend on progress with new technology and would be a long-term prospect.