I just got done reading a Baseball Prospectus book that analyzed the best pennant races of all-time. The conclusion was that many times a manager can actually mess up a team in a pennant race.
They listed a few actions that seemed to be consistent with bad pennant race managers: over-reliance on aging players (Juan Pierre), refusal to play young players (even after they've proven to be better than the older players), using an ineffective pitcher on a regular basis (Peavy), and continuing to bat underperforming players high in the lineup (Dunn).
Its funny that Guillen exhibits all these characteristics of a bad manager. He repeatedly gives the most playing time/at bats/innings pitched to the least productive players. He is too inflexible.