In his first-ever Instagram post, the actor shared a video of himself reading a 1989 Amazing Spider-Man comic featuring Mysterio, and captioned it, "I just realized I'm not playing Spider-Man."
Sony declined comment, but the movie's official Twitter account did tweet the video, giving the casting idea even more credibility.
In the comic, Mysterio tries to convince Spidey he caused a man's death, The Hollywood Reporter reports, noting that it's unclear if this plot will carry over into the film.
In the comics, Mysterio is the alternate name of Quentin Beck, a movie stuntman and special-effects wizard who uses his skills in creating illusions to commit crimes.
Spider-Man: Far From Home stars Tom Holland, not Gyllenhaal, in the lead role, and comes out in July 2019.
First published Dec. 5, 3:17 p.m. PT.
Update, 4:54 p.m. PT: Adds that Sony declined comment.