The biggest difference between the 2019 team and the 2021 team are the 2019 had Chris sale and David Price. But as this was not exactly vintage Sale (147 IP, 4.40 ERA) nor vintage Price (107 IP, 4.28 ERA), it is not that difficult to imagine those numbers being replicated or exceeded by Richards and Perez. That bullpen also relied heavily on Colten Brewer and Marcus Walden, two pitchers who will have to earn their way back on to the 2021 team. And this could get even more difficult if/when the Sox sign Sawamura.
Replacing Mookie with Verdugo is a loss, but replacing Holt with EHern should be a gain. Its not hard to see Devers as being better, despite his brillaint 2019 season.
but more important is the AL East competition. Are the Rays (25.1 fWAR) still the best pitching staff in the AL, like they were in 2019? Are the Yankees still a top 10 pitching staff, especially considering the durability of their starters? The Jays' staff should be better, but that does not mean they are good. Those 3 teams account for over 1/3 of the Sox opponents in 2021.
That said, any of these teams could still improve substantially if they choose to spend on some of the remaining arms. But then so could Boston...