(Reuters) - United Airlines (>> United Continental Holdings Inc) has no immediate interest in purchasing 777X aircraft from Boeing Co (>> Boeing Co) but instead views the plane's predecessor, the 777-300ER, as a potentially good fit for its network, the carrier's Chief Financial Officer, John Rainey, told Reuters in an interview Monday.

While Rainey has said in the past that the 777-300ER interests United, he said on Monday that the aircraft could be a quick fix in markets where United needs additional seats to satisfy demand so customers are not forced to book on United's competitors. United currently is considering whether to swap Boeing 787 Dreamliners it has on order for its first 777-300ER aircraft, which can fit more seats.

The swap would be a victory for Boeing, which has long insisted it can sell the 100 777-300ERs it plans to produce per year before it shifts toward building the 777X, starting in 2017. Analysts have expressed concern that Boeing might have to cut prices or production as it makes this switch.

Rainey said United is not interested in the more expensive 777X, to be outfitted with General Electric (>> General Electric Company) engines, "at this moment, but there was a point in time when I would have given the same answer about the (777)-300ER, so things do change."

He clarified that a deal would exchange planes on order without adding new purchases to United's books. The airline has yet to make a decision on the swap, which would not be a "massive" conversion, Rainey said.

Still, Rainey added that 777-300ERs could arrive sooner than the similarly sized Airbus (>> AIRBUS GROUP) A350-1000s that United has ordered for first delivery in 2018. It also could complement United's plans to take two, still-larger Boeing 747 aircraft out of its fleet this year.

"One thing that you could see is that we could consolidate all of the 747 flying in one area, and that would have a cascading effect with the 777-300ER in other markets and the 787 somewhere else," Rainey said.

"This isn't necessarily a statement about the 787," he said, adding that the routes on which Dreamliners fly, such as Denver-Tokyo, "work very well."

Rainey said United does not currently have an interest in new 747-8 purchases.

Instead, he said, United would like to see a replacement for the discontinued Boeing 757 aircraft, which "has a particular sweet spot in our network," adding that the Airbus A321neo equivalent had "issues" but he did not elaborate.

(Reporting By Jeffrey Dastin in New York; Editing by Ken Wills)