We're all more than used to Marvel movies including special cameos or teasers during the mid- and post-credit scenes. Sometimes these are mentions of big team-ups that are to come, sometimes these are appearances by surprise characters. Shang-Chi and the Legend of the Ten Rings included a mid-credit scene in which Captain Marvel and Bruce Banner made an appearance, but this scene featured a detail that fans were quick to point out.
Bruce Banner, played by Mark Ruffalo, was not in his spectacled charming Professor Hulk form! This was interesting, despite the fact that Bruce was still wearing the same outfit he wore at the end of Avengers: Endgame. Given that he seemed to have achieved happiness with his green side, why give it up? Apparently, one Marvel fan, who goes by the name "cosinecasino" on Reddit, may have already figured out this curious mystery, and its connection to She-Hulk.
According to this theory, Bruce's decision to revert back to his human form may have something to do with the upcoming She-Hulk series coming to the Disney+ in the near future! The series is set to introduce Jennifer Walters, played by Tatiana Maslany, who is Bruce Banner's cousin. In the comics, Walters gets attacked and shot by crime boss Nicholas Trask, who had a bad brush with her father. After Bruce gives her a life-saving emergency blood transfusion, Jennifer gains the ability to turn into She-Hulk.
It has been confirmed that Mark Ruffalo will appear in the She-Hulk series. Redditor cosinecasino believes that Bruce will have to revert back to his human form in the series in order to give Jennifer the same blood transfusion that made her She-Hulk in the comics. As the redditor says, "In order for Prof Hulk to help out, he has to switch to Banner. This also allows Disney to use the cheaper, non-mocapped version of him in the show except for the scenes that need him. Banner showing up here is just a nod to the show + helps organize it in time."
This is an interesting theory and definitely holds some weight given what we already know about Hulk's powers in his MCU-specific incarnation. In the Joss Whedon-directed The Avengers of 2012, it's been established that regular materials (such as medical needles) wouldn't be able to pierce Hulk's powerful skin. This means Bruce wouldn't be able to save Jennifer's life when in Professor Hulk form if it came down to that decision. It could be possible that the decision would be in haste considering that in the comics it's a medical emergency.
This is also interesting, if Marvel doesn't stray too far from Jennifer's comic book origins, it seems like we will see less of the charming Professor Hulk in the MCU. This would also mean that Jennifer's transition into She-Hulk has already started before the events of Shang-Chi, putting the She-Hulk TV series among the growing number of Phase Four titles to take place just a few months after the events of Avengers: Endgame. Of which we already have the critically acclaimed WandaVision, The Falcon and the Winter Soldier, and Loki on Disney+.
In any case, Mark Ruffalo is just happy to play Hulk in the MCU. There's a high chance that the explanation for Bruce not wearing his green skin in Shang-Chi won't be revealed until the premiere of She-Hulk on Disney+, so until then, we have no way of proving this theory to be true or not. That being said, however, compared to some of the other whackier theories, this one seems to hold much more weight on the "hows" and "whys" of Banner's appearance. Shang-Chi and the Legend of the Ten Rings is playing in theaters now. This theory comes to us from Reddit.