Stevenson was pretty good last year but RBs come and go all the time and there's only a handful who are any good past their first 4 years (Zeke is probably one, at a greatly reduced workload). They get a lot of punishment and you need to be in top shape to make guys miss, have any breakaway speed and drive through them. It shouldn't be surprising when a guy doesn't keep improving.
The Mac INT was just everything that he has been doing wrong this season all rolled into one. Bailing on the pocket early, making a bad decision / trying to be a hero instead of throwing it away when it was 2nd and 4 in FG range, and then screwing up his footwork into the throw (doing a weird stutter step) which led to horrible accuracy.
I also want to call out the poster upthread who gave the injury gods a goat. Fuck that. Injuries happen in the NFL, the Patriots have always been about having depth, and nothing excuses being THIS bad. They have their QB1 playing, 3 of their top 5 WRs, their top two TEs. This is a 7 win team with perfect injury luck and if some injuries make the Krafts realize that they're actually a bad roster without much top-end talent and questionable coaching up and down, the franchise will be better for it.