Spoiler alert, that’s likely due to Avengers: Endgame, in which Steve Rogers passed on the mantle of Captain America to Falcon.
Stan’s comments appear to indicate audiences will see Falcon tackling that responsibility, but he didn’t leave any clues as to how Sam may take it.
Sebastian Stan’s comments also make it sound as though Bucky will play a supporting role in the MCU Disney+ adventure, as he stated a bulk of the series will center around Anthony Mackie’s Sam.