But his actual rate stats are pretty similar. So did he get worse or did the rest of the team get better, and he doesn't stand out as well as before?
Considering Bergeron and Marchand's relative numbers are also worse this year other than GF%, despite most considering them having their best seasons, and Boston jumping huge in the standings, I have to think it's the latter. The best players on a team don't get worse and the team itself gets considerably better. The team simply has better depth than before, and with Krug in particular, the Chara- McAvoy pairing is much better than recent top pairings with Chara, which has sank Krug's relative numbers.
This is always the issue with using relative numbers. Sure, being on a great or terrible team needs to be taken into account somewhat when considering a player's pure rate stats, but more than anything it's only comparing them to the other defenseman pairings. It's why Ghost's relative numbers also need to be put in context as Philly's bottom 3 defensemen are terrible.