The record is secondary to the problems at the ML level and the logical argument for replacing Farrell.
1. The Red Sox are horrible in-division. So either the ML scouts, John Farrell, or both are doing something wrong here. This team has a substantially worse record in-division than out of it when the AL East isn't exactly a powerhouse where Baltimore, Toronto, Tampa, and New York go beating up on all out of division opponents. At times this year the AL East has looked like the worst division in baseball. This is a clear sign of an organizational problem that needs to be corrected, and if you don't believe it is the source of some sudden new scouting deficiency that happened to coincide with Cherington and Farrell arriving on the scene it's pretty hard to view it as anything other than Farrell getting out-managed routinely by every other manager in the division. If so that is simply inexcusable.
2. The Red Sox have done a very poor job introducing young players to the majors. John Farrell's job includes bringing up young new talent, finding a comfortable environment for them, and getting the most out of them in addition to his veterans. He has been manager of the club during what I would think anyone would agree is the greatest recent wave of young home grown talent. He has produced fewer standouts from that bumper crop that Tito did with FA cast offs and mid-tier prospects. His only two (bordering on three with EdRod) successes were premier blue chippers. He hasn't been at the helm when any young solid but not elite talent made the next step to become a legitimate contributor. His closest claims of those sorts are three journeymen who in the mid/late 20's got over the hump and became valuable role players (Salty, Nava, Holt).
3. He frequently costs the team wins by favoring veterans in all things. He plays obviously toast/ailing/hobbled vets over young players already promoted to the active roster. He lets veterans continue to flail away against statistically hopeless split match-ups to show loyalty. When it comes to making out the lineup card Farrell is on a polar extreme of the "win games today v. win hearts/minds for tomorrow" debate and extremists need idealized scenarios to succeed.
4. As others have said, a huge part of Farrell's claim to fame is his pitching pedigree. That has been nowhere to be found post-2013. At this point whether he and Nieves actually "fixed" Lester and Lackey or those two established veterans self corrected isn't really important. We're on the second year in a row where Farrell has been unable to get projected or better production from his staff.
Farrell is good at not rocking the boat. I think he was the perfect guy for the 2013 roster built almost exclusively with proven veterans with little young talent pushing for time. Given that kind of roster and getting the right seasons out of the players on that roster Farrell can win a title, obviously. He won't rock the boat, maintain an even keeled clubhouse to the end, and avoid the disastrous blow ups that can derail that kind of team. As the 2015 Sox pursue their second last place finish in two years and third in four I at least feel like maybe some boat rocking would do the club some good.
He just isn't the guy to manage a club trying to execute a youth movement. He isn't getting it done in many other elements of his job either, so there isn't some major benefit to living with that shortcoming.
Beyond that the club needs to consider why they ever thought Allard Baird was a worthwhile hire. The club's FA signings have, in direct correlation with his arrival, gone into the toilet. They also need to ask themselves if Ben Cherington is really a quality Major League General Manager, or just so damn good as a director of player development that you feel like you have to promote him, when doing so is just fulfilling the peter principal and dooming him to failure. The farm is stocked but god damn have they made some spectacularly bad decisions at the ML level, and not the "wow, that looks bad in hindsight" kind but the "this doesn't look good day one" followed by "wow, this looks awful one year out" kind.