Avengers: Infinity War - 35 Easter Eggs & References Explained
33. Banner Does A Silver Surfer
After the opening sequence, when Thanos lays waste to the Asgardians (and presumably lets half of them escape before he destroys the ship), Heimdall is able to send the Hulk to Earth to warn his fellow Avengers about Thanos' impending arrival (before he's killed).
Hulk crashes into the Sanctum Sanctorum, transforming back into Banner after the impact to pass on the message. And the way he smashes into Doctor Strange's headquarters mirrors how the Silver Surfer - the comics' herald of Thanos' conquest - lands in the sanctum himself.
Obviously, with Silver Surfer off the cards for Marvel until the Fox deal goes through, they had to go with a replacement character.