"That's a James Gunn thing. We never intended to put Adam Warlock in Infinity War," Russo said, referring to the Guardians of the Galaxy writer-director.
"We have so many heroes we have to service that you guys love and give them screen time, because you want to see the hero and villain have an emotional connection," he explained.
"You wouldn't have a connection to [Warlock]," Russo said, because the artificial cosmic being "doesn't have a connection with Thanos and built-up story."
Adam Warlock was hinted at in a mid-credits scene in Gunn's Guardians of the Galaxy Vol. 2 by way of a golden cocoon.
In the Marvel comics, Warlock, originally dubbed 'Him,' was created by the Enclave — a small pack of scientists out to create an artificial and perfect human in order to establish a powerful new race of beings under their control, which they could then use for their own nefarious purposes.
Once the being reached maturity and escaped its incubation chamber, it later recognized its creators as evil and escaped. He would later don the name Adam Warlock.
The character, whose powers would come to include a bond with the Soul Gem — the Soul Stone, one of the six Infinity Stones, in the Marvel Cinematic Universe — had ties to the cosmic side of the Marvel universe, including Thanos, both beings described as part of the "cosmic norm."
Warlock played a pivotal role in the Infinity Gauntlet storyline — partial inspiration for Avengers: Infinity War — and its two comic book crossover sequels, The Infinity War and The Infinity Crusade.
Part of Warlock's comic book story goes to Benedict Cumberbatch's Doctor Strange in the movie adaptation, who was established in his 2016 solo movie before appearing a second time in Thor: Ragnarok.
Avengers: Infinity War co-writer Christopher Markus said he and writing partner Stephen McFeely considered introducing the character, but held off because of the already expansive cast.
Bringing in Warlock would have been "a massive backend," Markus told ET, saying you "practically have to make an Adam Warlock movie to introduce him. He just can't walk on screen."
Despite Warlock being left out of Infinity War, Gunn told SlashFilm the golden man "will be part of the future Marvel cosmic universe," saying he'll stand as a "pretty important part" once he joins the shared Marvel Cinematic Universe — likely in Guardians of the Galaxy Vol. 3, expected to reach theaters in 2020.
Avengers: Infinity War is now playing.