Tellingly, there were few mistakes in his rookie season and he quickly developed a reputation for taking a methodical approach to a weekend.
“He's been unbelievable,” Brown told Speedcafe of Piastri's impact.
“We know he was mega fast but, with any rookie, especially one that took a year off, so you think there'd be some rust there.
“I didn't see any rust, or he rinsed it off pretty quickly.”
Piastri's debut was cut short due to a technical problem in his car, but raced wheel-to-wheel with team-mate Lando Norris in just his second race.
He then scored his first world championship points in Australia in a chaotic encounter that saw three red flags.
His early performances prompted Brown to describe him as a future world champion, a view shared by McLaren team principal Andrea Stella.
“What was most impressive about his was his patience, his attitude, his technical ability, and his ability to build up to a weekend,” Brown explained.
“I think a lot of young drivers try and win Friday. Then end up crashing and then you're on your back foot all weekend.
“Oscar was one session at a time, knew it didn't count until Q3 in qualifying, so would experiment. He would learn. He would listen. He would talk to Lando. He would look at data.
“Sometimes he was right there on Friday, sometimes, you know Monaco, he was a half a second off. Come qualifying, he was right there.
“That was impressive,” he added.
“To have a driver with that type of maturity of building into a weekend…
“His race craft is getting stronger and stronger, which is a hard part with the lack of experience.
“These tyres are tricky to manage over a race, and he just build into the weekends wonderfully.”
Piastri enters his second season of F1 with McLaren this year with a contract in place to keep him in Woking until 2026.
Team-mate Lando Norris has recently extended his stay in Woking with a new ‘multi-year' deal – his previous contract was set to expire at the end of 2025.