Stephen McFeely and Christopher Markus have both confirmed that Professor Hulk (Mark Ruffalo) was supposed to appear for the first time in Avengers: Infinity War (2018) during the final battle scene. They revealed this during the panel at SDCC 2019 where Markus said that the scene was due to unfold as follows: during the battle, Hulk was due to break through the Hulkbuster armour and go one to one with Cull Obsidian. Or, as he put it, ‘[kick] Cull Obsidian’s ass’. However, it was removed from the film in the editing process.
There’s a distinct lack of Hulk in the final cut of Infinity War; he appears in the first scene where he goes head to head with Thanos on the Asgardian’s ship. But when he’s later humiliated and scared by the defeat he faced, he refuses to come out again, no matter how much Banner begs him to help them. So, Bruce has to take up the mantle by donning Tony’s Hulkbuster armour so that he can get involved in the Battle of Wakanda.
Professor Hulk is the culmination of the two – Bruce Banner’s brains paired with the Hulk’s strong body. The union of the pair was revealed in Endgame with Banner telling us expositionally that it took 18 months of study in a gamma lab for the pair to be able to live side by side together. This makes more sense than for it to happen spontaneously which is inevitably why the cut was made.
Markus said, “It didn’t work! It was completely the wrong tone for that moment in the movie. It was this moment of victory while we were headed toward defeat.”