Mercker is 6-5 with a 5.12 ERA this season in 25 games, 18 of them starts. Since July 1, Mercker is 4-2, with a 3.99 ERA. He is also 1-0 with a 2.53 ERA in four appearances against American League teams this season.
Mercker was 11-11 in 1998, leading the team in wins in his first season in St. Louis after signing as a free agent. For his career, Mercker is 60-58.
Mercker was expected to join the Red Sox on Wednesday in Minnesota.
Red Sox general manager Dan Duquette said the addition of Mercker should strengthen the team for its stretch drive. Boston entered Tuesday night's games tied with Oakland for the AL wild-card spot.
"He's a versatile pitcher who has the ability to be effective as a starter and as a reliever," Duquette said. "He has had success throughout his career late in the season and also has extensive postseason experience."
It wasn't clear if Mercker would initially start or relieve. The Red Sox are a starter short since placing right-hander Bret Saberhagen on the 15-day disabled list Monday with a weak throwing shoulder.
For his career, Mercker is 4-2 with a 3.16 ERA in games in September and October, including 3-0 with a 2.70 ERA last September.
Mercker pitched in the playoffs during the 1991-1995 seasons as a member of the Atlanta Braves. He is 0-1 with a 2.25 ERA in the postseason.
In exchange for Mercker, the Cardinals got left-hander Mike Matthews and catcher David Benham.
Matthews, 25, is a career minor leaguer with a lifetime 43-48 record. He began this season at Buffalo, Cleveland's Triple-A affiliate. He was sent to Double-A Akron in June and was traded to the Red Sox on Aug. 3, pitching for Double-A Trenton. Overall, he is a combined 0-5 with a 7.47 ERA this season.
Benham, 23, was drafted by the Red Sox in the 12th round last June. He is hitting .238 with three homers and 11 RBIs in 33 games for Class-A Sarasota.
The Cardinals assigned Matthews to Double-A Arkansas, and Benham to Class-A Potomac.
The team also said that Larry Luebbers will replace Mercker in the starting rotation and will pitch Wednesday at Montreal.
©1999 The Associated Press. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed