Florida, in the thick of the NL East and wild-card races, made the deal a few hours before baseball's 4 p.m. ET trade deadline.
The 38-year-old Rhodes is 2-1 with a 2.86 ERA with 26 strikeouts in 22 innings. He has a lifetime record of 77-61 with a 4.28 ERA in 689 games.
Rhodes' career appeared in jeopardy when he underwent Tommy John surgery in May 2007. But he came back ahead of schedule this year, returned to the Mariners in mid-April and pitched effectively.
Rhodes was the Mariners' main lefty setup man after they traded George Sherrill to Baltimore in an offseason deal for Erik Bedard. But Seattle's season quickly fell apart and with so few save opportunities for the bullpen, Rhodes wound up as a situational lefty.
Mariners vice president and general manager Lee Pelekoudas said the opportunity to trade a veteran for a prospect was the right move as his team points toward next season.
"We felt the opportunity to acquire a young pitcher made sense as we look toward 2009 and beyond," Pelekoudas said. "Pedro Grifol, our minor league field coordinator, has known Gaby for several years and believes he has a chance to help us in the future."
The 21-year-old Hernandez went a combined 5-8 this season with a 6.47 ERA in 17 starts for Triple-A Albuquerque and Double-A Carolina.