Leclerc, starting just his second season in Formula 1, earned his promotion after achieving strong results at Sauber. He comes in to replace Kimi Raikkonen and offer Ferrari a different dynamic in 2019.
The man from Monaco is expected to become one of the sports superstars. His best time was just 0.010s away from Vettel's best time across the two weeks in Barcelona.
Brawn, an ex-Ferrari man himself, doesn't think Leclerc will beat Vettel but reckons he'll cause some trouble.
"I'd be surprised if I'm honest, Sebastian is very good," Brawn said in an interview on Motorsport.com
"If it unsettles him a little bit, I think that's the thing that the team will have to manage, not necessarily Charles beating him, but the fact that he could unsettle him."
Brawn is one of the many people who expect Leclerc to produce magic in the future and is the young driver to watch this season.
"I think he's the number one in that respect. He's had a very good first year, and he's now in a top team. Talking to some of the people I know at Ferrari, they've been very impressed with his approach and attitude. I think he's pretty special," Brawn added.