To placate the Yankees' concerns about his durability, Rivera followed a strength and conditioning program in the offseason, instead of throwing. Torre said that he would try to reduce Rivera's workload during the 2003 season to minimize the injury risk to his closer. However, Rivera suffered a groin injury before the season began, causing him to miss the first month. After returning on April 30, he pitched well in the season's first half, saving 16 games in 17 opportunities. His save on June 13 against the St. Louis Cardinals secured the 300th career win for starter Roger Clemens. Rivera slumped early in the second half; over one stretch, he blew five of eleven save opportunities, but he rebounded to convert his final 15 of the season. He finished the 2003 regular season with a new career best in ERA (1.66), along with 40 saves in 46 opportunities. In the AL Championship Series against the arch-rival Boston Red Sox, Rivera had one of the most memorable postseason performances of his career; in the decisive Game 7, he entered in the ninth inning with the score tied 5–5 and pitched three scoreless innings, his longest outing since 1996. He became the winning pitcher after Aaron Boone hit an eleventh-inning walk-off home run that clinched the Yankees' series victory and advanced them to the 2003 World Series. Rivera celebrated by running to the pitcher's mound and collapsing in joy to thank God, as Boone rounded the bases and was met by his teammates at home plate. Rivera was named the AL Championship Series MVP for recording two saves and a win in the series. The Yankees lost the World Series to the Florida Marlins; Rivera saved five games and allowed only one earned run in 16 innings pitched that postseason.