Joe Russo thinks that the injury that Hulk picked up during the events of Avengers: Endgame is permanent and irreversible.
After the heroes manage to pull off Operation Time Heist in Endgame, Iron Man forges a new Infinity Gauntlet through the use of nanotech. Hulk wields the gauntlet to reverse the Decimation after everyone but Thor agrees that he is the one who is most likely pull this off. He succeeds, but the act severely damages his arm.
In a recent interview with ComicBook, Joe Russo suggests that this particular injury isn't going away. He points out that Thanos too had suffered from a similar injury after his snap, and that one too wasn't going to heal. According to the Endgame co-director:
He's lost an arm. He lost Natasha. That's not coming back. He's damaged himself. I don't know. It's interesting. That's permanent damage, the same way that it was permanent damage with Thanos. It's irreversible damage. His arm, if you noticed, is a lot skinnier. It's blackened. So, he loses a lot of strength there.
So, that means Hulk is going to have to live with a damaged arm for the rest of his life? Well, maybe not. Joe then does a U-turn and insists that the Marvel Cinematic Universe doesn't lack in extraordinarily genius characters, one of whom might help Mark Ruffalo's character to fix his arm injury. In his words:
But who knows? There's a lot of smart people left. Maybe someone helps him repair that. Maybe someone gives him a new arm. I have no idea where that character goes from here. The nice thing is we didn't have to pay attention to where it goes after this, we just try to tell a satisfying ending.