They have plenty of arms, even with the injuries, but you can't throw 'em out there 1 per inning losing 8-1, or even 5-1. Especially if you're playing 7 games in 7 days. You look up in a 4-3 or 4-4 game, you got nobody good when you need them.
How did Farrell get to this? Well, most all managers have the same regimen these days. 3-4 bullpen guys per game. But a few of them are a bit more flexible than that. They will throw a guy out there for 2-3 innings if he's going good--to save the bullpen. Bochy, Showalter, Madden. Some of the better managers.
That's the way it used to be in baseball, when long relief was a reality and starters had to earn starting through the bullpen. That's not the way the Red Sox do things anymore, to their detriment.