Detroit dealt the left-handed pitcher and cash to the Arizona Diamondbacks for right-hander Billy Buckner, who was sent to Triple-A Toledo.
The 28-year-old Willis was designated for assignment on Sunday, giving the Tigers up to 10 days to make a move with him. He was making $12 million this year.
"We are paying a substantial portion of that salary," said Detroit general manager Dave Dombrowski, adding that a couple clubs showed interest.
Willis was 1-2 with a 4.98 ERA in eight starts and one relief appearance this season. He is expected to start Saturday night in Arizona against Colorado.
"I’m just excited to get a fresh start," Willis said on a conference call.
He joins a team that had lost eight in a row entering Tuesday night’s action and ranked last in the majors with a 5.89 ERA.
"He’s had extreme success, he’s had a period of struggles, and he’s hopefully coming out of it," Diamondbacks general manager Josh Byrnes said. "He’s always been known as a big personality and we’ve encouraged him to be himself and breathe some life into this team, which we need."
The former NL Rookie of the Year, All-Star and 22-game winner helped Florida toward the World Series championship in 2003, but failed to remotely regain that form in two-plus seasons with the Tigers.
"It was a great place to play baseball," Willis said. "I just didn’t get it done."
Arizona manager A.J. Hinch talked to Willis shortly after the trade went down.
"He’s an energetic guy who loves to play," Hinch said. "He wears his heart on his sleeve and has a lot of charisma. He had a lot of success early in his career for his play and his personality and his approach. He brings a fresh outlook to our team, and a fresh personality."
Willis joined the Tigers as part of the trade that brought Miguel Cabrera to Detroit after the 2007 season, and signed a three-year, $29 million contract extension.
Willis was 2-8 with a 6.86 ERA in 24 games — 22 starts — and was put on the disabled list twice last season with what the team called an anxiety disorder, a diagnosis he disputed.
One of the better hitting pitchers in baseball, he said he was thrilled to be returning to the National League.
"Sometimes for an athlete it’s definitely rejuvenating that someone else wants you and someone else believes in you," he said.
Willis, who already had moved from Florida to Scottsdale, Ariz., is the first lefty on the Diamondbacks’ staff.
Asked if the pitchers’ recent inconsistency might be an added burden on the bullpen, Byrnes said, "It can’t make it any worse." The Diamondbacks’ bullpen ERA of 7.70 is by far the worst in the majors. Arizona lost 5-4 Monday night in Los Angeles on a ninth-inning balk to fall to 20-32, 11½ games behind NL West leader San Diego.
"It’s probably as bad as we could have imagined," Byrnes said, "and we’ve got to figure out a way to get out of it."
Buckner was 0-3 with an 11.08 ERA in three starts, which Dombrowski said were "not good," for Arizona this season. The 26-year-old righty has spent four years in the majors, going 4-6 with a 6.40 ERA in 13 starts last season for the Diamondbacks.
He started the season at Triple-A Reno after failing to win a spot in the rotation this spring and failed to take advantage of a recent opportunity to pitch for Arizona.
"He gives us depth at Toledo," Dombrowski said.
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