I think you can interpret Cherington's bad run one of two ways.
A) He had the wrong strategy. The Lester and Lackey (and, later, Cespedes) trades prioritized present value instead of building for the future. He signed a bunch of expensive free agents when he should have saved the money for something else down the road.
B) He had the right strategy but executed it poorly by choosing the wrong players. We should have traded Lester/Lackey/Cespedes for different major league players who could help immediately. We should have signed different free agents.
If it's A), then it's really puzzling that the new regime immediately traded a huge prospect package for an elite player at a position with a very limited half-life, and committed $217 million to a free agent. So I think it's pretty clearly B). Which makes it important to suss out how much of that was his fault as opposed to the fault of the scouting department, and what (if any) better options existed.
In any case, I continue to think that it's all well and good for Cherington to have been pushed out in the name of accountability, but I think there's a difference between pointing out that this was a terrible run, which is obviously true, and arguing that said terrible run reveals something about the flaw in his ideological priors or the "right" way to build a winning team.
If the prospect package is headlined by prospects in AA/AAA (such as Josh Bell or Joc Pederson, to name two options rumored available at the 2014 deadline), there's no significant difference between choices A and B.
Although these players can't be immediately added to the MLB "core" they should only be about 0-2 years away from solid complementary contribution. That's the point of getting an Eduardo Rodriguez, although there's obviously also the ever-present threat of getting an Edwin Escobar instead.
It's only when the team trades MLB talent for more highly-rated prospects in the A-ball minors that the distinction between a retool and a rebuild actually becomes important.
MLB Players acquired 7/2014 - 4/2015: total = 1.4 fWAR in 2015
7/2014 --- (Cespedes), Craig, Kelly
8/2014 --- Castillo
9/2014 --- none
10/2014 - Uehara
11/2014 - Sandoval, Ramirez
12/2014 - Porcello, Masterson, Miley, Hanigan
1/2015 --- Breslow (re-signed), Ross, Ogando
2/2015 --- Miley (extended)
3/2015 --- none
4/2015 --- Porcello (extended)
MLB Players traded away 7/2014 - 4/2015: total = 17.5 fWAR in 2015
7/2014 --- Peavy, Lackey, Lester, Gomes, Miller
8/2014 --- Johnson
12/2014 - Cespedes, Middlebrooks, Webster, de la Rosa, Wilson
If winning "now" was the goal, something went dreadfully wrong, no matter what the "right" way to build a winning team may have been.