Rosas was crucial in Sydney's 17-point win over Melbourne, kicking five goals before halftime en route to victory at the SCG on Sunday.
The 24-year-old had flown under the radar when he arrived at the Swans last year, with the spotlight on the club's high-profile trade for former Carlton spearhead Charlie Curnow.
Curnow's trade cost key forward Will Hayward and a raft of first-round draft picks, as well as Ollie Florent in a separate deal.
But for Rosas and pick No.62, Sydney parted ways with just pick No.51 and No.69.
The 24-year-old, a grandson of AFL Indigenous Team of the Century member and Australian Football Hall of Famer Bill Dempsey, had played just 49 games over six seasons for the Suns before making the move.
In front of 40,673 fans against the Demons on Sunday, Rosas marked himself as one to watch, becoming only the fifth Swans player in the past 10 years to kick seven or more goals in a game.
Rosas has scored 13 goals so far for the ladder-leading Swans (7-1), behind Joel Amartey and Curnow, who both have 20 majors.
Sydney coach Dean Cox described Rosas as capable of doing things other players can't, but the matter was simpler for the humble Darwin-born forward.
"I got lucky," Rosas laughed.
"It was just falling into my lap. I was like, 'Oh, they (the Demons) can't really do nothing about it'.
"I just came here (to the Swans) to try to put my best foot forward. The support around me helps. I'm just playing my role."
While Rosas was quick to brush off praise, Melbourne coach Steven King could only admit to awe.
King crossed paths with Rosas as an assistant coach at the Suns from 2021 to 2023.
"I actually gave him a bit of love before the game, so I probably shouldn't do that again," King joked.
"I maybe would have loved a bit better if he'd kicked two, but seven's a stretch.
"He only needs a sniff, and he made us pay.
"I'm really proud of the way he's settled in Sydney, and his career's gone to another level."Â
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