Andretti won the 1978 world championship with Lotus and is regarded as one of the greatest racing drivers of all time.
The Italian-born American also won the Daytona 500 and Indianapolis 500 in a professional career that ran from 1964 to 1994.
After hanging up his helmet from full-time competition, he continued to compete in the 24 Hours of Le Mans, winning his class in 1995.
“I’m an Oscar Piastri fan because I’ve watched him closely and I think Zak Brown is very, very clever in choosing his drivers,” Andretti told GPBlog.
“I think Zak Brown right now has the perfect team set up. Either one of the drivers are very capable of not only setting a quick time on pole, but also managing a race and being able to win.”
While Lando Norris sits second in the drivers’ championship behind Max Verstappen, with an outside chance of the title, Piastri has been the revelation of the season.
He’s won two races and proved unflappable under pressure, points which have marked him out as a future superstar of the sport – and potentially a world champion.
However, Andretti wouldn’t say if he felt the young Australian was better than his team-mate.
“I don’t like to go there in that sense,” he said.
“But he has the qualities of a champion. It’s not that he’s hot and cold. He’s always there.
“It looks like he will support Lando because Lando is in a better position points-wise. But at the same time, he’s a racer. He wants to do his thing as well.
“For the short period of time that he’s been in Formula 1, he’s showing that he’s definitely a future champion.”
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Andretti has links with McLaren, his son, Michael, having raced for the squad in 1993 alongside Ayrton Senna.
Michael Andretti also has business interests alongside McLaren Racing CEO Brown through their involvement in Walkinshaw Andretti United.
McLaren currently leads the Formula 1 constructors’ championship by 41 points over Red Bull Racing with six races (and three sprints) remaining.
That position has been the result both of a consistently competitive car and a driver pairing that has delivered.
It has allowed the squad to reel in Red Bull Racing, which at one stage enjoyed a 114-point advantage, leaving the Papaya team with a target on its back, according to Andretti.
“I have got to give congratulations to McLaren quite a bit as to how well they have progressed through the season,” the 84-year-old said.
“It’s showing. It hasn’t just happened once. They’re there. Right now, it looks like they’re in form.
“They’re the ones to beat at the moment.”
Formula 1 returns this weekend with the United States Grand Prix from the Circuit of The Americas, opening practice beginning at 12:30 local time on Friday (04:30 AEDT Saturday).