The simple fact is that winning is what creates a good mental attitude and the expectation of winning the next game. And losing, especially losing more than the year before, has the opposite effect.
That said, you only have to watch the games to see that the wins and losses are earned and rarely the result of some indefinable malaise.
Exhibit A in this argument is Chris Sale, who I think is the most professional pitcher in Red Sox uniform I have seen recently. Yet he is having a horrible season, and I refuse to believe it's because he pitched 6 fewer innings in ST this year than last year (2018). He has changed. His control isn't as good, his fastball has lost some steam, and his changeup ain't working although his slider is. It may also be that he is more comfortable with Leon behind the plate than Vazquez.
Exhibit B is Mookie Betts. He's getting $20M and is line for a huge free agent contract, and his numbers are way down from last year, especially with RISP. I would argue that opposing pitchers and especially coaches have figured some things out and know a lot more this year about how to get him out, especially with RISP. Last night was just one more example.
Exhibit C includes Bogie, Devers, Vazquez, JBJ, and maybe even Chavis, all of whom are hitting better or much better than last year. Plus Devers has even gotten better defensively. No way, no how are those five not looking forward to excelling in the next game and doing what it takes to win.
Exhibit D is the rotation writ large. As I have already detailed, ST did not kill them because ERod pitched the normal 15 innings this year and is pitching worse than last year and Price is pitching better than last year but pitched 6 fewer innings in ST than last year. Only Sale is a lot worse than last year and pitched 6 fewer innings in ST, but I would argue that he is worse because his fast ball has lost some zip, his control is not as good, and his changeup ain't working but was last year. My point is that our $88M/year rotation (Sale will get more than $15M next year) just maybe wasn't a good investment.
Exhibit E is the bullpen, who are paid peanuts--less than $10M for the whole bunch of them. The Yankees, on the other hand, have spent on a lot less for the rest of the rest of their roster and so have not one closer but in fact, have spent $40M on Ottavino, Britton, Germain, and Chapman, all of whom have higher WAR's than the Yankee starters except for Tanaka, whose WAR is 1.8 to Ottavino's 1.5. Their bullpen is the envy of MLB and gives that a big edge in almost every game they play. DD simply could not afford to do that for the Sox bullpen.
Exhibit F is that the Red Sox in the John Henry era have already demonstrated that they rarely follow a great season (win the WS) with a really good one. There is always a let down. And guess what? Last year was only the greatest season in Sox history with 108 wins in the regular season and taking 11 of 14 games in the postseason against the Yankees, Astros, and Dodgers.