WARNING: The following contains spoilers for Shang-Chi and the Legend of the Ten Rings, now in theaters.
Shang-Chi and the Legend of the Ten Rings lets fans know that this new phase of the Marvel Cinematic Universe is unlike anything seen before thanks to its world-building and its cast. While the film introduces new characters and items like the Ten Rings, it also reintroduces classic characters. A great example of this is in the movie's mid-credits scene, which reintroduces Bruce Banner. However, rather than appearing as the Smart Hulk, he instead appears in his human form. This change is both surprising and might be a hint towards an important MCU development.
In Avengers: Endgame, Bruce explains that he spent 18 months in the Gamma lab following the Hulk's defeat by Thanos. In that time, the two personalities came to an agreement and decided to share their brains and brawn into one entity. Unfortunately, this was the form that also took the brunt of the radiation when he used the nano gauntlet to bring back half of all life with the Infinity Stones. With Banner returning to his human form in Shang-Chi, there are a few possible reasons, including the Hulk persona being dormant or even dead. However, the character's comic roots could also provide some insight.
Banner has consistently tried his best to get rid of the Hulk aspect of his personality in the comics. Though his desire is logical, it almost always fails because stopping the Hulk means Bruce has to erase a part of himself. However, this hasn't stopped him from trying on more than one occasion. For example, during Fear Itself #7, Bruce resorts to seeking aid from Dr. Doom. With his aid, Doom successfully separates the two entities, putting Banner's consciousness into a cloned body. While a success, it didn't last because Banner eventually wanted to become the Hulk again.
The Onslaught event also had Banner and Hulk separated after the Phoenix turned off the Hulk's Banner persona. By punching Onslaught with all of his strength, the psionic backlash managed to make Hulk and Banner separate entities. However, like all great Hulk stories, this accidental separation didn't last forever. Banner was also cured of the Hulk in the early years of his comic with The Incredible Hulk #223, where the Green Goliath was drained of his Gamma radiation and reverted to his human form. However, following a battle and extra dose of radiation, he was back to being big, mean and green.
Banner and Hulk have rarely seen eye to eye, and their merger in the MCU seemed like a miracle when it occurred. However, as his constant cures and solutions in the comics have proven, it's not meant to last. For now, there has been no explanation as to why Banner and the Hulk have separated, but from a film perspective, it could be due to the Hulk's rage that his form was injured while using the gauntlet. Alternatively, he could have also been drained by giving blood to his cousin, Jennifer Walters, making her the She-Hulk.
In the MCU, Bruce Banner's connection to the Hulk hasn't been completely explored. Therefore, it's hard to speculate what lengths must be taken to separate the two. That being said, with the MCU advancing in sciences and concepts, it's possible that any of his comic cures could have been implemented to revert Banner. Whatever the case may be, that cure could open many scientific doors for the MCU or lead to destructive results depending on the state of the Hulk.
Check out Mark Ruffalo's Bruce Banner in Shang-Chi and the Legend of the Ten Rings, in theaters now.