The injury Smart Hulk suffered from Avengers: Endgame being miraculously fixed in Phase 4 can be explained if Marvel Studios decides to bring back one character from Avengers: Age of Ultron. While the rest of the surviving heroes spent their time differently on the heels of Thanos' snap in Avengers: Infinity War, Bruce Banner used his figuring out a way to combine the best of his two personas. This resulted in the introduction of Smart Hulk in the MCU who has the raw strength of the original green-rage monster and consciousness of his human alter-ego.
With this unique combination, Smart Hulk was the only character who could've pulled the reverse snap successfully without dying (although Thor also volunteered). Despite having difficulty withstanding the collective power of all six Infinity Stones, Smart Hulk was able to accomplish his goal. While he didn't suffer any fatal injuries, it left his right arm badly burned. On the heels of Thanos' defeat, Endgame didn't have time to reveal the severity of the injury after Smart Hulk's reverse snap. But, it's clear that he was still hampered by it as he wore an arm while he helped Steve Rogers jump to the Quantum Realm to return the Infinity Stones, and Phase 4 is exploring it further.
Related: Phase 4 Can Now Explain Endgame's Missing Hulk Mystery
In Shang-Chi and the Legend of the Ten Rings, Banner had reverted back from Smart Hulk to his old self, but his arm was still visibly injured and in a sling. However, in She-Hulk he'll be back as Smart Hulk, yet Hulk's arm no longer has the Avengers: Endgame injury. It's unclear just what happens or how Smart Hulk fixes his arm, but one of the most intriguing possibilities would be for Dr. Helen Cho to help. The world-renowned
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