This was a fascinating James comment on several levels. He doesn't usually comment on the Red Sox, especially off-field. But the circumstances here were common knowledge.
So, this is an acknowledgement that off-field behavior / team chemistry can *hurt* a team on the field. If chemistry can affect a team negatively, it's not clear why state of mind cannot affect a player positively.
Sabermetricians (BJOL company excepted) are agreed that a player can be a choker, so to speak, and then he washes out of the big leagues, or goes 7-19 for the Yankees, or something. Odd that they resist the idea of an UPside coming from better-than-average state of mind. This is a paradox at Baseball Prospectus and Fangraphs, that they'll be happy to assume the downside of the mental game but will strictly rule out the upside.
The September collapse in Boston that year ... hm. Going into Sept. 1, the Sox had won 6 of 8, improving their record to 83 and 52, leading the AL East by 1.5 games. By 'best team' James may be referring to the tremendous depth up and down the roster ... Clay Buchholz looked like a budding star but had to battle Andrew Miller for the 5 SP slot. Papelbon was a star closer. Daniel Bard was sensationalizing the setup role in one of the early monster bullpens. Erik Bedard and DiceK scrabbled to get a few innings. We're not even talking about the Big Four, Beckett - Lester - Lackey - Wakefield.
Offensively that team scored 875 runs. AGone had an Edgar year at .338/.410/.548. David Ortiz had a .398 OBP and .554 SLG. Jacoby Ellsbury was setting himself up for huge money by going .321/.376/.572 with 39 stolen bases and 119 runs scored; he looked like he could become the best player in baseball. Dustin Pedroia had a great year.
They had Kevin Youkilis, J.D. Drew, Carl Crawford, Jarrod Saltalamacchia, Josh Reddik, Jason Varitek, Mike Cameron. Break up the Sox.
But on Sept. 1, A.J. Burnett beat Jon Lester as the Sox dropped a rubber game to the Yankee$.
On Sept. 2, Andrew Miller was wiped out for 6 runs in 1.1 innings while Derek Hlland fired a 2-hit, 0-walk shutout* - obviously looking like a "hangover" game.
The Sox split the next two games and then on Sept. 5 they lost a walkoff game, 1-0 in 11 innings. Boom, they were -2.5 out.
They went 7-22 in September to wallow like a garbage scow -8 games out, and this was the best Sox team in 100 years per James. That was the season the "fried chicken in the clubhouse" thing erupted into a melodrama. Here's an even more interesting link on it. Generally speaking, when a pro sports coach gets Wakamatsu'd I consider the coach innocent until proven guilty. Like Pedroia said, the players make the money and it works best when they police themselves.