Actually, you're wrong because the defense has been the same all season long, including when the Sox led the AL in wins back in July. What went downhill was the rotation, which finally forced Cora to send Richards and Perez to the bullpen, which has also struggled. Indeed, so has the hitting. Let's not forget that in the 2d half the Sox have been hit hard by COVID, which has had it's biggest effect on pitching and hitting, not defense.
Indeed, right now with Iglesias at 2b so that Verdugo can play LF, Kike CF, and Renfroe RF, the defense is possibly better than earlier in the season. Even Dalbec has shown signs of mastering 1b--but, sadly, not Schwarber.