I am not a fan of trying to attribute the difference in defense by putting ratings on the individual players for 2 reasons.
First of all, those ratings are VERY subjective, and I think in many cases are colored by how the team is playing this year versus last year.
Second, the problem has had a lot more to do with team play than individuals IMO. You can have better defensive players, but if they don't know the system or click together, then you are not going to have a good defensive team.
Last year, the C's came together as a well oiled defensive machine down the stretch. They became greater than the sum of the parts. This year, they have been less than the sum of the parts.