There are managers who do help their team (Bochy, Francona, Maddon, a couple others) ... and a few who actively hurt their team (those Matt Williams seasons in Washington, Dusty Baker). The rest are in the middle.
Farrell was crucial in 2013 when the Red Sox just needed a normal human being to occupy that position after the Bobby Valentine season. The most direct impact he can have is managing the staff - and while how he has managed relievers has not had amazing results, I am not sure the raw material has been great either. At least this year he seems to have a reliable back of the pen.