Well that was a pretty sweet week.
Gotta say RDM has been great with his player selection, but the one thing he changed (that was a pretty glaring one) that AVB was too stubborn to, was starting Mata underneath the striker on a more permanent basis. Under AVB, Mata predominantly started on the left wing, then would naturally drift into the center where there was less space because some other Chelsea player had started there.
This left a big 'ole hole on the left wing that made Cole feel compelled to try to supply some sort of width with. Now the hole is at left back, trying to be covered by the adjacent CB and dropping midfielder(s). Thats why our defense was so porous under AVB.
Now with RDM, Mata starts in the center and someone else (Kalou/Sturridge/Ramires) starts on the left wing, not only supplying width themselves, but allowing Mata more space to create, and Cole feeling less responsible for providing that width himself and keeping better track of attackers, letting the CBs as a result work together better.
This is also why Torres has improved, because he has Mata consistently supplying chances for him, along with a more stretched defense. It was a small change with a big effect.
/soapbox