Green Lantern never appeared in Zack Snyder's version of the DC Extended Universe, but his films already set up the introduction of Hal Jordan. Snyder's first DC movie, Man of Steel, arrived in 2013 and was layered with Easter eggs for a wider DC universe. This includes references to Bruce Wayne and Lex Luthor through Wayne Enterprises and LexCorp logos. In these cases, Snyder was able to pay off the teases with both characters joining the DCEU in Batman v Superman: Dawn of Justice.
As the second film in his overall DC plan, Batman v Superman: Dawn of Justice is responsible for setting up the DCEU's future Snyder wanted. The movie featured the introduction of Wonder Woman, teased her solo movie, and included cameos by Aquaman, Cyborg, and Flash ahead of Justice League. These four plus Superman and Batman are the six members of the Justice League in Snyder's DCEU and will be the center of Zack Snyder's Justice League. But, it was widely expected that one of the Green Lanterns would join the story at some point.
Snyder never got far enough in the creation of the DCEU to cast Hal Jordan or John Stewart, but that didn't stop him from subtly setting up the introduction of a Green Lantern. The setup is particularly for Hal Jordan and the use of a supporting character from his comics. In both Man of Steel and Batman v Superman: Dawn of Justice, audiences got to see Captain Carrie Farris (Christina Wren). This name should register on some level with fans of Green Lantern's comics, as it is a clear nod to Caroline 'Carol' Ferris. She is the longtime love interest to Hal Jordan in the source material who is also an expert pilot.
Even with this Green Lantern connection in the comics, it is unclear if Farris would be tied to Hal Jordan if he ever debuted in Snyder's DCEU. Farris had a minor role in both of her film appearances and Snyder has proven time and time again he's not afraid to go against the source material. She might be an important part of Hal's story in the past, but Snyder might never have made that connection. After all, he killed Jimmy Olsen after only one scene. Although, Man of Steel showed she already has an interest in superheroes as she called Superman hot, so a romance with another intergalactic hero could be right up Farris' ally.
While Snyder hasn't revealed if Farris will appear in Zack Snyder's Justice League, the movie could include additional setup for Green Lantern. The history lesson sequence of Darkseid's first attempt to invade Earth is expected to include a Green Lantern character, just as it did in the theatrical cut. There is also speculation that Hal Jordan or another more recognizable member of the intergalactic force could appear by the film's finale. And with the DCEU movie's four-hour runtime, there is always the chance other Easter eggs will be included that reference Hal Jordan, the death of Abin Sur, and so on.