"This is a now move with an experienced guy," Marlins president of baseball operations Larry Beinfest said. "We've got to make some decisions shortly."
Robertson, 32, has been with the Tigers since 2003, and last year, the left-hander was 2-3 with a 5.44 ERA. He appeared in 28 games with six starts.
The arrival of Robertson adds a lefty to what was shaping up as an all right-handed rotation.
Beinfest added that Clay Hensley, a non-roster invitee who had been lined up as the fifth starter, will likely make the team. Hensley now projects to join Burke Badenhop as a long reliever.
"Robertson is going in the rotation. It is very likely that Hensley will make this team," Beinfest said. "We will figure out exactly how the back end of that rotation looks. We traded for Nate with the thought that he will go into the rotation."
Robertson has appeared in six Grapefruit League games for the Tigers, with three starts. He's 2-1 with a 3.66 ERA, with 19 strikeouts and seven walks in 19 2/3 innings. He had been in competition for a rotation spot with Dontrelle Willis and Jeremy Bonderman.
The Marlins had been working on this trade for almost a week. Recently, Mark Wiley, the former Florida pitching coach who now is an advisor, was following the Tigers, watching Robertson.
The Marlins traded Robertson to the Tigers in 2003 as part of the Mark Redman trade, a move that helped Florida win the 2003 World Series. Redman went 14-9 with a 3.59 ERA in 29 starts during the championship season.
2010 Spring Training - null
Sights & Sounds
Spring Training Info
Before being finalized, the deal had to be granted the approval of the Commissioner's office, because more than $1 million was being exchanged. Robertson is signed for $10 million this season, and the Marlins are paying the minimum of $400,000, with the Tigers picking up the rest.
Robertson is expected to join the Marlins in Jupiter, Fla., on Wednesday.
"He is left-handed, which is something we didn't have here in the rotation," Beinfest said.
Florida's rotation now shapes up as Josh Johnson, Ricky Nolasco, Anibal Sanchez, Chris Volstad and Robertson.
In recent years, Robertson has dealt with some injuries. Last summer, he had surgery on his left elbow, and in the offseason he had surgery to repair a torn groin muscle.
Robertson also brings playoff and World Series experience to a Marlins team that has postseason aspirations.
"He's been successful, and we know the guy," Beinfest said. "He's done well, and we know the type of person that he is."
The Marlins entered Spring Training with two lefties vying for rotation spots. But Andrew Miller and Sean West were sent down to Triple-A. West will miss about three weeks with a back injury. Miller, meanwhile, is a candidate to join the rotation at some point in the season.
Bringing in Robertson also means it is likely Rick VandenHurk and Tim Wood will open the season in the Minors. VandenHurk, who has been a starter in his career, recently has been pitching out of the bullpen. And Wood has enjoyed a strong Spring Training.
Both will likely start off at Triple-A New Orleans.
In 202 career games, Robertson has made 169 starts over eight seasons.
Voss is a promising lefty reliever, who was in big league camp early with the Marlins. An eighth-round pick in 2007, Voss was reassigned to Minor League camp on March 11.
"This just builds natural depth in the organization," Beinfest said. "It just adds another pitcher, and adds some depth."