Throwing a no-hitter doesn't automatically make a pitcher great, since it's just one game. Just like hitting one big home run in a clutch situation doesn't make you a great hitter.
But, when you can say, by the age of 24, that you have beaten cancer, won the clinching game of a World Series, thrown a no-hitter, thrown a complete game shutout against the Yankees in Yankee Stadium, then you can say that you're well on your way to becoming great.
He's already being considered by many experts to be one of the best lefties in the league. His on-field accomplishments will speak for themselves.