So, what changed for John Farrell between 2013 and 2014? Essentially the same assets, exactly the same manager and drastically different results... So, with the same assets, the same manager went from winning the World Series (sorry, let's pause while I throw up in my mouth a little...) to dead last in the AL East.
Now, there are objectively bad managers (Cito and his absolute hatred of rookies, Farrell and his torrid love affair with sac bunts... etc) who still manage to win despite obvious, glaring mistakes because they have either a good team, have the stars align or have a mediocre team who plays well over their heads for a season.
What I'm saying is that certainly, making completely moronic decisions (batting your best hitter 6th, sac bunting in the 2nd inning...) can certainly HURT a team, there aren't any strategies that managers can employ to get more out of a team than the players can provide over the long haul.