During Haaland's debut campaign, he scored 52 goals in 53 games and was arguably the best player in the greatest Brittish team ever. Not to mention how many individual accolades Haaland have earned after his debut season.
Nobody is questioning Javi Martinez as a genuine Bayern Munich legend despite he had only 1 great season for them. And that was his debut campaign in 2012-13 season when Bayern won their first Treble. At that time, Javi Martinez was easily regarded as the best DM in the world.
So there's already a precedent for it.
If Haaland gets a career-ending injury tomorrow and is forced to retire from the game, would you consider him as a Man City legend?