From today's WSJ:

Slugger Albert Pujols was supposed to start seeing better pitches after his St. Louis Cardinals traded for Matt Holliday to bat after him. Lest pitchers walk Mr. Pujols too often, Mr. Holliday, a sound hitter in his own right, would have more at-bats with players on base. But since the move, Mr. Pujols is batting .200 with zero homers, his longest drought of the year. Mr. Holliday, though, is batting .541, with three home runs.

Here's the abstract of Bradbury and Drinen in the J of Sports Econ:

Past studies estimating the marginal revenue products of baseball players have assumed individual players' hitting performances to be independent of teammate spillovers. However, the baseball community's widely held belief in "protection"—that a good (bad) player can improve (diminish) the hit probability of the batter who precedes him in the batting order—violates the assumption of the independence of batting outcomes. In this paper, the authors identify two possible hitting externalities in baseball. Using play-by-play data the authors find evidence contrary to the protection hypothesis—the quality of the on-deck hitter negatively impacts the preceding hitter—though the magnitude of the effect is very small.