Before landing the role of Doctor Sivana in David F. Sandberg's upcoming Shazam! film, actor Mark Strong appeared as the villain Sinestro in 2011's Green Lantern, which starred Ryan Reynolds in the titular role. Although that movie released before 2013's Man of Steel, which is considered the first film in the DC Extended Universe, Strong has now asserted that Green Lantern is in fact part of the DCEU.
In an interview with Screen Rant, Strong was asked how his role in Shazam! affected the place of Green Lantern in the DCEU. "It is part of the DCEU," Strong responded. "But, you know, my version of Sinestro I think is going to be rebooted, as they say, no doubt when the new Green Lantern comes out, which I think they’re doing. But as I said and I mean it, I felt really... it was a little interrupted because Sinestro was gonna go into the second movie and become the yellow Sinestro that we know and love and cause havoc. It got cut short by the fact that they never made a second movie of that particular incarnation of Green Lantern and Sinestro. But, thankfully, I’ve now got a chance to maybe exercise my evil credentials with Doctor Sivana."
Strong's statement clearly asserts that Green Lantern is part of the DCEU, and it's clear that he's talking about the film and not Hal Jordan or Sinestro. Plus, if he is talking about either character from the film, asserting that they're canon in the DCEU would still indicate that the film from which they come is part of the cinematic universe.
This definitely doesn't mean that Green Lantern is actually part of the DCEU, as it's unclear how much Strong knows about Warner Bros.' plans with Green Lantern and that film's place in the cinematic universe's canon. It's entirely possible that he misspoke or misunderstood the question, thinking that Screen Rant was asking about the existence of the character in general.
Green Lantern is often considered one of the worst superhero movies of all time, and the film only made $219 million against a budget of $200 million. Although there were plans for a sequel, those were scrapped, and there are currently plans to reboot Hal Jordan with a Green Lantern Corps. film.
Arriving on April 5, 2019, director David F. Sandberg’s Shazam! stars Asher Angel as Billy Batson, Zachary Levi as Shazam, Mark Strong as Dr. Thaddeus Sivana, Djimon Hounsou as the ancient wizard Shazam, Grace Fulton as Mary Bromfield, Jack Dylan Grazer as Frederick “Freddy” Freeman, Ian Chen as Eugene Choi, Jovan Armand as Pedro Peña, Faithe Herman as Darla Dudley, Cooper Andrews as Victor Vásquez and Marta Milans as Rosa Vásquez.