Warning! SPOILERS Below for Justice League.
Justice League finally brings Green Lantern to the DCEU - but how exactly does it set up the Corps? The original marketing for Justice League proudly declared "Unite the Seven!" However, the roll call of the team in the movie boasts only six of the World's Greatest Superheroes: Batman (Ben Affleck), Wonder Woman (Gal Gadot), Aquaman (Jason Momoa), The Flash (Ezra Miller), Cyborg (Ray Fisher) and Superman (Henry Cavill). Someone's missing, a prominent DC Comics superhero typically regarded as a core Justice League member.
As it turns out, Green Lantern does make an appearance in Justice League after all, but it's not Hal Jordan, the most famous Emerald Gladiator and a founding member of the League in the comics. That's right - in the DC Extended Universe, the Justice League is now officially formed without the aid of Hal Jordan or any Green Lantern. In fact, there is no Green Lantern on Earth in the modern day era of the DCEU. The Justice League trailer contained a line of dialogue from the movie's villain Steppenwolf (Ciarán Hinds) that isn't in the final cut of the movie about Earth having "no protectors, no Lantern..." But Earth did have a Green Lantern to protect it long ago.
Related: Where Is The DCEU's Green Lantern?
Five thousand years ago, Steppenwolf launched his first attempted conquest of Earth. The fearsome being known as the End of Worlds sought to use the Unity of three Mother Boxes to transform our world into another version of his homeworld Apokolips. To beat back this invasion, an alliance to defend the Earth was formed between the Amazons (Wonder Woman's people), the Atlanteans (Aquaman's people), all of the tribes of Man, the Greek Gods and a Green Lantern. They fought the Parademons of Apokolips led by Steppenwolf in a brutal battle and won.
The Green Lantern who fought for Earth was a caped Emerald Gladiator who used his Power Ring to create a giant hammer to swat Parademons to shreds. However, Steppenwolf soon killed the Lantern with his battle axe. Upon the Lantern's death, his Power Ring immediately flew off his finger past Steppenwolf to look for its next wearer.
From the brief glimpse of the Green Lantern in the "history lesson" Wonder Woman told Batman, the Lantern seems to resemble one from DC Comics lore: Yalan Gur. In the comics, Gur was the Green Lantern of Space Sector 2814 (where Earth is located) about two thousand years ago. Yalan Gur was considered one of the greatest Lanterns of his time by the Guardians of the Universe. The Guardians removed the impurity in Gur's Power Ring that made it vulnerable to the color yellow, allowing him to become more powerful, but the power corrupted Gur and he began remaking the worlds in Sector 2814 to resemble his desires. The Guardians then punished Gur by depowering his Ring, making it vulnerable to wood, which led to Yur's death. Yalan Gur's life energy bonded with his Power Battery and ended up being part of the Starheart, which gave Alan Scott, the Golden Age Green Lantern his power.
It isn't clear from his few seconds of screen time whether the Lantern in Justice League is actually Yalan Gur or another character from the same race. Indeed, it's likely his presence is meant as a cool Easter egg and a shout out to the hardcore Green Lantern fans in the audience. However, we do know for sure there has been no Green Lantern around to protect the Earth since 2013.
Only Superman fought to save the Earth from General Zod's Kryptonian invasion, a violent event that would otherwise demand the intervention of a Green Lantern. Similarly, the absence of a Green Lantern and the death of Superman were key reasons why the Mother Boxes awakened after five millennia and summoned Steppenwolf back to Earth to complete his conquest. Each space sector is actually meant to have two Green Lanterns assigned to it, yet no Green Lantern joined the Justice League in fighting Steppenwolf.
The whereabouts of the Green Lanterns in the DCEU is a huge question fans want answered. There is a Green Lantern Corps feature film mooted, rumored to be a 'buddy cop' movie that will be about Hal Jordan teamed with another Lantern (presumably John Stewart), which was announced by DC Films for a potential 2020 release. If that movie does happen, fans will hopefully finally get the answers to what happened to the Green Lanterns and why they seem to have abandoned Earth.
Green Lantern is also at the top of the list of superheroes who fans want to see join the League in Justice League 2. Unfortunately, how much longer we'll have to wait to see the Green Lantern we know and love shine his emerald light in the DCEU is anyone's guess.