It's Sandy Leon.
Now before you send me to internet jail for that statement, hear me out. I'm not suggesting that having Sandy behind the plate would have cured the rotation's horrendous start. I'm also not suggesting his veteran leadership in the clubhouse would have made a noticeable difference. (And I'm definitely not advocating for his bat). In fact, I'm in the camp that agreed it was time to move on from Leon and finally commit to Swihart.
The huge mistake the Red Sox made was WHEN they made this decision. Although he hasn't been the greatest contributor on the field, Leon has been a core member of this team for several years, and a highly valued asset to the pitching staff. Guys understand that this is a business, and sometimes their favorite teammates have to be released. It's just the nature of the profession, but that's no consolation.
Dombrowski had to know Sandy wasn't going to be a part of the 2019 squad, and should have made that unpopular call in February at the latest. There hasn't been enough of a market for Leon (or Vazquez or Swihart) to bring anything of value in trade, and even a good Spring Training wasn't going to change that fact.
Personnel--wise this is practically the exact same team that just set a franchise record for wins and cruised to a World Series victory. They're playing flat and lifeless ball right now, and nowhere moreso than the starting rotation. If they had had all winter to get used to the idea of Vazquez and Swihart as the top two, I believe this team would more closely resemble the one that opened last season 17-2. Seeing their close friend strung along for months and then cut right before the season started has led to an unsurprisingly lethargic start. Releasing Sandy in January is the one, easy move the Sox could have made that would have had the biggest impact on their record.