Speaking at the Airline Economics conference in Dublin, Laderman said the airline would also look at the smallest model of the Boeing 787 Dreamliner and the Airbus A321neo in the same stage its fleet replacement plans
I think big airlines are talking to all their major suppliers all the time. We shouldn't think much of it. Now the responsible guy is pointing to the A321NEO in relation to its fleet replacement plans. Laderman is the guy who signed up for the 737-10 at the Paris Airshow.
United sees a potential new Boeing jet available in 8-10 years.
Deliveries starting 2028 is probably too far away for United The 757/767 fleet are passing 20 yrs (on average..) already. http://www.airfleets.net/ageflotte/United%20Airlines.htm
The 737-10 seems not the solution for all their network requirements ( Caribbean, South America, Western Europe). The 787-8 is under consideration too, but has twice the weight, range and price. So probably a different application.
United ETOPS 757 at Manchester
It wouldn't be very surprising to have United considering A321s because American, Delta, Spirit, Frontier, Jetblue, Hawaiian and Alaska have been moving to A321s and they are operating in the same marketplace.
United would probably focus on the improved A321 NEO ACF to give them medium range / premium cabin flexibility.
A321NEO ACF prototype, Jan 2018.