Jenson Button must "see himself as the number one driver" at McLaren as he bids to seize control of the team and a second World title.
With Lewis Hamilton off to Mercedes this season, Button has been teamed up with Sergio Perez.
By far the more experienced of the two, Button has spent more years with McLaren than Perez has spent in Formula One and as such the Brit has become the team's unofficial number one.
And although McLaren are insisting both drivers will get equal treatment, Stewart reckons this season offers the perfect opportunity for Button to stamp his authority.
"It's not a question of equal footing this year," the triple World Champion told Press Association Sport .
"You've someone in Jenson who knows the engineers, telemetry people, knows the business, and who has been there, seen it and done it.
"So I think he can do it again (win the world title) with the right equipment, and there cannot be a better supplier of that equipment than McLaren.
"They are about as good as they come. They've got the facilities, so many assets, so yes, he can do it if the car is good enough and he is psyched up enough.
"He no longer has to keep up with Lewis and that's a mental thing, so he has to see himself as the number one driver, and that McLaren are depending on him."
Stewart, though, concedes there are still a few questions about Button's ability and whether he can string together a run of results.
"When he wins races they are beautifully-controlled races.
"You just know he is not going to fly off the race track or make huge errors of judgment. He is very solid.
"Whether he has what it takes to be a serial winner (with McLaren), the jury is still out because he hasn't done that yet."