My guess is, if Britton, like Sale, was on the Sox, you'd be pointing to an irreversible decline and near certain doom.
2014-2016:
1.38 ERA
2.40 FIP
0.909 WHIP
9.2 K/9
2.4 BB/9
3.91 K/BB
0.3 HR/9
2017-2019
2.74 ERA (2.30 in 2019)
3.92 FIP (4.09 in 2019)
1.320 WHIP (1.24 in '19)
7.3 K/9 (7.3 in '19)
4.6 BB/9 (4.8 in '19)
1.58 K/BB (1.52)
0.5 HR/9 (0.6)
Only his ERA is better than his EVAN last 3 year norm.
Ottavino is having a great season, especially with his ERA (1.41), but many of his numbers have declined from 2018:
WHIP: 0.991 to 1.275 (Very big rise)
FIP: 2.74 to 3.37 (significant rise)
BB/9: 4.2 to 5.5 (significant rise)
K/9: 13.0 to 12.4 (very slight)
K/BB: 3.1 to 2.3 (significant)
H/9:4.8 to 6.0 (25% rise)
HR/9: 0.6 to 0.7 (slight but still a 17% rise)
I'm not trying to poo-poo their records, but there are sign that they have not been as great as their ERA's suggest they have been.