Norris had told reporters on arrival at the Dutch Grand Prix that he felt he was not performing at the level of a world champion.
The Briton is second in the drivers' standings but 78 points behind Red Bull's triple world champion Max Verstappen with 10 races remaining.
Stella said Norris was measuring himself against perfection and tended to see the half-empty part of the glass, but it was normal even for champions to have races where they could have done better.
"In reality, I think he definitely has the potential of a world champion and he's performing at world championship level," said the Italian, who worked previously at Ferrari with greats Michael Schumacher and Fernando Alonso.
"Lando is definitely world championship material. He performs already at levels that are seen in the past in some of the great champions I have the luck to work with."
"He can win world championships. That's a statement, that's capital letters."
Stella said Norris took responsibility for calls, particularly in Canada and at Silverstone, which the team should really be blaming themselves for.
"If the team had operated at higher standards then Lando would have more points in the championship," declared the Italian.
"As a team, we kind of appreciate that he puts some of the pressure on himself or he raises his hand and says it was on me, but I think I have enough experience to say 'Mate, don't worry, that's on the team, keep doing a good job, keep working to improve on your own opportunities'.
"But definitely we as a team have quite a lot of work to do as well."
McLaren are second in the constructors' standings, 42 points behind Red Bull.
"Numerically, from a driver's championship point of view, it looks like there's a big gap to fill and we are chasing Max Verstappen, so definitely he's not going to make it easy for us," he said.
"But we are excited to be in this position, and definitely Lando is not in this position because somebody helped him. Lando is in this position because he performs very strongly, and that's something that we should acknowledge and realise."
