I don't think it's a shift in philosophy. Rather, I think it's taking advantage of what's on the market to fill holes.
Last year, the Celtics shooting wasn't very good, and neither was their rebounding. The Celtics have attempted to fill those holes through several of their free agent signings, including Shaq, Wafer, Delonte, and to some extent J.O.
At the same time, there wasn't a ton that Danny could do about some of the loss of defense. Perk got injured, so the team's #1 priority immediately became to fill his hole. They did that by signing a pretty good defensive center in J.O. He's probably a downgrade, but that was unavoidable; there were no defensive centers better than Perk on the market.
Then, Tony left to find fame and fortune elsewhere. Danny's immediate reaction to that was to attempt to sign Ronnie Brewer, a very good defender. When Brewer chose Chicago, Danny re-signed Marquis (a guy whose defense Doc has raved about repeatedly) and signed Delonte, a good defender. Again, this may end up to be a slight downgrade, but it wasn't for lack of trying.
It's likely that the defense will be a little bit worse, especially until Perk comes back. However, the Celtics have filled enough other gaps such as shooting and rebounding, so hopefully the Celts will be a better team overall.