Joss Whedon is juggling a lot in the upcoming sequel “Avengers: Age of Ultron.” Despite the first movie having more superheroes than any film in recent memory, he’s upping the ante by adding three more characters (Scarlet Witch, Quicksilver and Vision) as well as a brand-new villain (Ultron). With so much on his plate going into the movie, the director had to let go of a character who's been vital to the Ultron comic storyline.

For those unfamiliar with Marvel comic books, Ultron was not created by Tony Stark (Robert Downey Jr.) and Bruce Banner (Mark Ruffalo). By this time in the source material, Hank Pym, also known as Ant-Man, is already an established member of The Avengers. The brilliant scientist creates Ultron in a failed experiment, and that’s how the villain comes to  life. Ultron gains power and becomes one of the biggest villains the superhero cadre has ever faced. Obviously, since the Paul Rudd-starring “Ant-Man” movie doesn’t come out until after “Avengers: Age of Ultron,” that story had to be retouched for the silver screen.

“Of all the heat I’ve ever taken, not having Hank Pym was one of the bigger things,” Whedon confessed in an interview with Empire Online. “But the fact of the matter was, Edgar had him first, and by virtue of what Edgar was doing, there was no way for me to use him in this.”

Whedon is referring to former “Ant-Man” director Edgar Wright (“The World’s End” and “Scott Pilgrim vs. The World”), who'd been working on an “Ant-Man” movie since 2006, well before Whedon was brought into the mix to do the first “Avengers” film. Although he had a very specific vision for the movie, which he’d been working on for almost eight years by that point, Marvel announced Wright would step down from the project, citing “differences in their visions of the film.” Still, by that time, the writing for “Avengers 2” was mostly done and Hank Pym wasn’t going to be around.

As hard as it was for Whedon to abandon the source material he grew up reading, he admitted to Empire Online that not including Pym in the sequel was the right choice anyway.

“Ultron needs to be the brainchild of the Avengers, and in the world of the Avengers and the Marvel Cinematic Universe (MCU), Tony Stark is that guy. Banner has elements of that guy – we don’t really think of him as being as irresponsible as Tony Stark, but the motherf----- tested gamma radiation on himself, with really terrible, way-worse-than-Tony-Stark results.”

This isn’t the first time the MCU has had to adjust its canon from the comic books. Thanks to its deal with Fox, no one in the MCU is allowed to mention X-Men or mutants. This will make Scarlett Witch and Quicksilver’s origin story in the movie unique -- in the comics, they’re the children of principle X-Men bad guy Magneto.

“Avengers: Age of Ultron” hits theaters May 1. While you wait, check out the latest full-length trailer for the upcoming sequel.