Whether the Marlins sink or swim this season will depend heavily on the mound performances of Mark Buehrle and Heath Bell, two free agents who came to Miami from pitching environments at opposite extremes of the hitter-friendly spectrum. For the first 12 years of his career Buehrle called the White Sox' U.S. Cellular Field home; over the past three seasons it was the easiest place to hit a home run in the majors. Bell, on the other hand, spent the last five seasons based in San Diego's Petco Park, the toughest run-scoring environment in the majors.
We don't yet know how Marlins Park will play, but it's fair to say that Buehrle should have an easier time than he did in his former home and Bell a more difficult one. With other pitchers who changed teams this winter, projections come more easily. Take Bell's former Padres teammate, Mat Latos, who was traded to the Reds in December. Latos will go from the best pitchers' park in the game to Great American Ballpark, which is average in terms of run-scoring but is particularly generous to the long ball—it had the majors' fifth-highest home run rate over the past three years. Latos was protected by Petco; just 8% of his fly balls allowed have gone for homers in his career. (The average starter's rate over the last five years has been 10.1%.) Even if the righthander pitches exactly as well as he did the last few years, he can expect his long-ball count—and ERA—to rise.
Two AL West emigrés face similar challenges. Yankees righthander Michael Pineda, a fly ball pitcher (44.8% of balls in play) as a rookie with the Mariners in 2011, was traded from a park that suppresses homers, Safeco Field, to Yankee Stadium, the second-best home run park in the AL and by far the best for lefthanded batters. Diamondbacks righty Trevor Cahill, who was acquired from the A's, moves from a good pitchers' park, Oakland's O.co Coliseum, to the less-friendly Chase Field in Phoenix, though that move is mitigated by Cahill's strong ground ball tendencies.
For a low-strikeout pitcher like Buehrle, who has not averaged five whiffs per nine innings since 2008, a change in the defense behind him can be just as important as park effects. The 2011 White Sox were next-to-last in the majors in turning batted balls in play into outs; that poor defense and Buehrle's pitch-to-contact approach added up to a whopping 221 hits allowed. The Marlins had an average defense last year—and should be better in 2012 with Jose Reyes taking over at shortstop. Between escaping U.S. Cellular and the Chicago defense, Buehrle may have had the best off-season of any player in baseball.
The counterpoint to Buehrle is righthander Hiroki Kuroda, who had a 3.07 ERA with the Dodgers last year while based in a very good pitchers' park in front of an above-average defense. Having signed with the Yankees, Kuroda now will make most of his starts in front of a short rightfield porch and an aging defense that was below-average a season ago. Like Latos, Kuroda could pitch exactly as well as he did in 2011 and see his ERA rise significantly.