Brie Larson was able to do a last-minute press push to plug her new Marvel movie -- this after the SAG-AFTRA strike ended -- but not even that could salvage ticket sales.

The actress -- who plays the lead in "The Marvels" -- got the green light from Kevin Feige and co. to hit the red carpet for the debut of their latest superhero flick ... not to mention a TV spot on 'The Tonight Show' with Jimmy Fallon, which squeezed her in in the 11th hour.

Her late-night appearance went about as well as you could imagine ... she talked about the movie, the director, the studio -- everything you could want mentioned ahead of a film's release. Unfortunately, though ... that doesn't seem to have boosted interest in her project.

Per box office estimates, "The Marvels" is on pace for an awful domestic opening -- by Marvel's standards -- with experts saying it's on pace to snag between $47-$52 million.

Between Thursday previews and Friday's full slate of screenings, the movie's only raked in $21.5 million ... marking the second-lowest Marvel opening day gross behind the original 'Hulk' movie that came out way back in 2008, before Disney even snapped up the IP.

The reason this is extra ugly is the fact that the first 'Marvel' movie that starred Brie -- mind you, this is a sequel -- actually cleaned up among theater-goers ... bringing in over a billion dollars worldwide in 2019. Now, 4 years later -- not nearly the same interest.

The question ... was it the content, or lack of marketing that torpedoed "The Marvels" here? It didn't really get a chance to get much chatter on account of the actors' strike that was going on up until just a few days ago -- so it's possible folks simply didn't know about it.