As early as late April, it became painfully obvious that the Red Sox' decision to bring their veteran starting pitchers along slowly in spring training was the wrong choice.
The rotation's inability to provide competitive starts in the first few weeks of the season resulted in a 6-13 record after three weeks. If that poor start didn't completely doom the 2019 Red Sox, then it certain dug them a substantial hole from which they never fully recovered.
It wasn't until May 8th that the Sox reached the .500 mark.
(To be fair, it's impossible to know whether the abysmal start to the season doomed them for the season. But it's far from a stretch to suggest that it was a major contributing factor).
The foundering start created a ripple effect for the rest of the pitching staff, too. With the starters unable (or more accurately, unprepared) to pitch deep into games, the burden fell to the bullpen to provide additional relief innings, and this domino-life effect created problems to the back end.
By the time the starters had built sufficient arm strength and the relievers recovered from overwork, it was nearly halfway through the season and too much ground had been ceded in the standings.
That miscalculation was costly enough - an ill-conceived bit of strategy that backfired and hurt them competitively.
But in recent weeks, that's become almost secondary.
Recall that the intent of the plan was two-fold: to ease the starting pitchers into the spring and early part of the season gradually, thus ensuring better performances in September and October as the Red Sox attempted to defend their title; and more broadly, this was designed to protect the pitchers' physical well-being, given what had been asked of the previous fall.