Hulk Was Injured In Ragnarok: Is That Why He Couldn't Beat Thanos?
Thanos beat Hulk in Avengers: Infinity War, but is that only because the Green Goliath was recovering from an injury from previous MCU film Thor: Ragnarok? Hulk's defeat at the start of the film was an important event, starting Bruce Banner on his performance-issues character arc and leaving the Avengers a very important man down as Thanos gained all six Infinity Stones.
The reason for Hulk's defeat is, especially when taken from a narrative standpoint, rather simple. He's the strongest Avenger (a fact confirmed in Ragnarok) and so for him to so effectively be dispatched by a villain previously hidden in the shadows highlights just how outmatched the entire team is. In The Avengers, Hulk was able to effortlessly defeat Loki (something the God of Mischief mentions before the fight); now this threat is even bigger.
All of this is surely leading to a rematch between Hulk and Thanos in Avengers: Endgame. Mark Ruffalo pitched a three-movie Banner arc to Kevin Feige ahead of Thor: Ragnarok, and after two movies where Banner and Hulk were unwilling to transform respectively, the natural progression is for them to come to peace with that idea and overcome what they previously couldn't. That peace is likely the Professor Hulk personality we've seen in marketing, while that problem to overcome is surely Thanos. But, even with the smarts of Banner, there's a question of how Hulk can possibly come back from such a definitive defeat.
The answer may lie in the first movie of the arc, Thor: Ragnarok. In the film's finale, Banner sacrifices himself to become Hulk and help stop Hela's rule of Asgard, fighting giant mythological wolf Fenris. During their tumble on the Asgard waterfall, Fenris grabs Hulk in his mouth and stabs one of his large fangs into his leg. While there's no noticeable injury on Hulk's leg in later scenes, the location of the bite is normally covered up by his trousers, and the moment stands out for the camera holding on the close-up for a long time during the carefully edited "Immigrant Song" sequence.
Thor: Ragnarok may be a rather comedic trilogy-capper, but it still works in Infinity War setup - best seen with Loki swiping the Tesseract, which the scene also lingers on for a little longer than necessary - so it's not out of the question that this Hulk injury is supposed to be important. It may be a subtle indication that the Hulk who fought Thanos initially wasn't up to full strength.
Hulk does, of course, have an accelerated healing factor that may render this thinking moot; it's entirely possible the Fenris bite was simply included to put Hulk in peril. But the focus put on his injury and the heavily-discussed connectivity between the three latest movies featuring the character leave it open as a potential explanation. Only time (and Avengers: Endgame) will tell.