There have been rumors for months that a Green Lantern would appear in Justice League. With the movie finally out, it's safe to say that those rumors panned out--sort of.
In a flashback scene describing an ancient battle between the forces of earth--Amazons, Atlanteans, and humanity--and the villain Steppenwolf, an unknown Green Lantern does appear.
Things escalated further when a Big Bang Theory contest to attend the Justice League premiere featured the Green Lantern logo alongside those for Superman, Batman, Wonder Woman, Aquaman, The Flash, and Cyborg.