Pennock missed most of the 1913 season with an illness, but was able to rejoin the team late in the season. In the 1914 season, Pennock posted an 11–4 win–loss record with a 2.79 earned run average (ERA) in innings pitched for the Athletics, and pitched three scoreless innings in the 1914 World Series, which the Athletics lost to the Boston Braves. Mack let Bender go after the season, naming Pennock his Opening Day starting pitcher in 1915. On Opening Day, Pennock threw a one-hit complete game shutout against the Boston Red Sox. However, as the Athletics struggled, Pennock's nonchalant playing style drew Mack's ire. Concluding that Pennock "lacked ambition", Mack sold Pennock to the Red Sox for the waiver price of $2,500 ($ in current dollar terms). Mack later regarded this sale as his greatest mistake.