I have watched and rewatched every game, and i have paid special attention to McCourty.
He has digressed in one technique/responsibility.
Last season he had a number of Ints from the position of trailing the deep WR. He drew those throws, and almost every time broke them up or picked them. That is 100% an issue of recovery speed. They wouldn't throw it if the WR didnt seem open.
This year those plays have resulted in completions, with Mccourty being 1-2 steps short of making a play on the ball.
Now, I am not downplaying the difference between 6-8 plays where you allow a long completion vs 6-8 picks or breakups, but aside from that technique, pattern and responsiblity his play has been remarkably similar to last season.
If you want to kill the guy for 6-8 plays over the course of a season, be my guest, but I would take the approach that 98% of what he has done has been just like his probowl rookie season, and he has an issue to correct. I do not know what mechanics have slipped, whether injuries affect it, whether he had a lazy offseason and never got in tiptop shape, or what it is, but I'm pretty confident a top all around corner who developed one flaw can correct it in time.