In exchange, Detroit will receive a player to be named later. The Tigers will scout a number of players in the Cardinals' farm system from a list the teams have agreed upon. They'll then have their choice of player later in the year.
Maroth, 29, has spent six seasons with the Tigers. In 2007, he is 5-2 with a 5.06 ERA in 13 starts. The lefty topped 200 innings in both 2004 and '05 before missing much of '06 due to bone chips in his pitching elbow.
"We're in a situation where we really need something like rotation help," said manager Tony La Russa. "We may have some relieving issues, and it allows one of our starters to go to the bullpen. I've talked a lot to the Tigers, and I think he can help us. I'm pleased that we were aggressive and we made a move."
Maroth will make his Cardinals debut on Monday night against the Mets at Shea Stadium. Brad Thompson, initially slated to make that start, will be pushed back to at least Wednesday. Thompson is available to pitch in relief over the weekend.
Probably best known for his 21-loss season in 2003, Maroth has been a reliable and effective starter in the intervening years.
"Mike is a young pitcher who we feel is only going to get stronger as he continues his recovery from last season's surgery," general manager Walt Jocketty said in a statement. "He is an established starter who is going to provide us with innings and help take pressure off of our bullpen. We think the change in leagues will also benefit Mike."
The St. Louis rotation has struggled mightily this year, and three Cardinals starters are currently on the disabled list -- Chris Carpenter, Braden Looper and Mark Mulder. Cardinals starters rank last in the National League in ERA at 5.52 and have averaged under 5 2/3 innings per start.
La Russa said he and Jocketty consulted with Tigers manager Jim Leyland and general manager Dave Dombrowski. All four men are friends with long-standing relationships.
Anthony Reyes went 0-8 and was sent to the Minors before being recalled over the weekend. Kip Wells, 3-11 on the year, was to be moved to the bullpen until Looper was sidelined. Todd Wellemeyer, a career reliever, has been pressed into starting duty, as has Thompson, who was expected to be the eighth-inning reliever at the start of the season.
Wellemeyer will pitch Tuesday in New York. Sometime in the coming days, the Cardinals will assess which of their starters to send to the bullpen or the Minors. According to La Russa, the Cards chose to make Thompson available in relief, rather than Wellemeyer, because Wellemeyer pitched more recently and thus Thompson is more rested.
Wells remains on track for his Sunday start due to his history of success against the Cards' opponent that day, the Phillies.
"As I explained to [Thompson], this doesn't mean that he's a reliever," La Russa said. "This means that Mike is pitching Monday and that's the day he pitches, so it enables us to use a guy who has been a reliever in the bullpen over the weekend and not feel like we're going to be short. And then once we get past the weekend, then we'll see what this means for Brad and the rotation."
Maroth has shown some distressing trends in his peripheral stats this season. His strikeout-to-walk rate has dropped below 1.00, and he's averaging a home run every 5.2 innings. Even so, in a rotation desperate for innings, his average of just over six innings per start is appealing.
Maroth is in the second year of a two-year contract that pays him $2.95 million this season. He is eligible for arbitration following the 2007 season, but cannot become a free agent until after 2008. He is the second starting pitcher acquired by St. Louis this week, following the signing of Tomo Ohka, who was released by the Blue Jays.
It initially appeared automatic that Ohka would join the Cardinals' rotation within a matter of days, but La Russa is preaching caution about the right-hander. Ohka is currently pitching at Triple-A Memphis.
"I'm assuming that he's going to have to pitch good enough to be good enough to pitch here," the manager said.
"It's just a whatever-number-of-starts look, then we make the assessment."
Meanwhile, the Cards are getting closer to adding two other starters back to their rotation. Carpenter continues to make progress, and is expected to take the mound in a Major League game before the end of July -- perhaps as soon as the team's first road trip after the All-Star break. Looper has begun playing catch and the Cards are very encouraged by what they are seeing.
Matthew Leach is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.