The only elite post defender on the Celtics is Perkins, Garnett doesn't have the build to be an elite post defender. But he routinely shuts down other tweaners like Dirk, Bosh and the like.
Garnett anchors the defense with his "help", I don't think I've ever seen a better help defender play the game. But to call him a lock-down post player just isn't accurate.
I partially disagree. Post defence is much more than playing straight up from behind - an option where Perkins is obviously better than Garnett, at least when defending the bigger low post players. But Garnett was the one defending Shaq, for instance. While I agree that what makes Garnett a great defender is essentially his help defence (that's what anchors a defence) and his amazing pick'n'roll defence, he can do a very good job guarding the low post.
Garnett also has an effective low post game offensively, but his build just doesn't allow him to play in the post consistently. It's the same on defense, if he had to guard the Dwights, Shaqs & Yaos of the world night in and night out he'd be worn out and the coaches know this. He's not a lock down post defender any more than he's back-to-the-basket post player.
You're definition of "post defender" seems to be pretty narrow. No, Garnett won't guard Shaq, Howard, or Oden often due to size/strength mismatches. But he can and has guarded every other great big man that I can think of in the post when needed, to good results.
And the "when needed" is a key aspect of this. KG has a lot of roles on this team: defensive quarterback, primary help defender, lead rebounder, lead post defender, shooter, pick-setter, post-presence offensively, offense facilitator/passer, etc. He generally does some combo of all of the above in every game, but not to the same degree. But I've noticed that when he needs to concentrate and ramp up any 1 particular aspect, he's been able to do that.
Focus: post defense. Since Garnett has been in Boston with the luxury to spend time/energy dedicated primarily to removing an opposing big man, he has shown that he can do it to pretty much anyone. Here's a Fave Five of outstanding offensive bigs (from low-post legends to the best perimeter scoring 7-footer ever) that KG has erased at key periods since becoming a Celtic:
1)
Locked down Tim Duncan in San Antonio. Perk started the game with the assignment and Duncan had 8 early points while the Spurs jetted out to a lead, then sometime in the second quarter KG switched onto Duncan and erased him from the game (2 points total for Duncan over most of the final 3 quarters) while Boston mounted a huge comeback to win.
2)
Locked down a hot Yao for the back half of the 4th quarter to end a Rockets game last January, leading a Celtics comeback victory.
3)
Locked up a scorching Amare (22 points at the half) for the whole 3rd quarter (0 points, 4 TOs) as a close game turned into a Celtics rout once Phoenix lost their best scorer during that key stretch.
4)
Erased Dirk on Sunday (Dirk had 6 points on 2-for-9 FG and 2-for-4 FT, 4 TO, 0 asts during the time when KG was on the floor). Dirk, by the way, was on fire, averaging 30.5 ppg in the 6 games before facing the Celtics.
5) Routinely
removes Bosh from games, almost at will, such as in the game 2 weeks ago when Bosh had 16 points with 8 minutes left in the 4th Q of a game where Toronto had a small lead. KG re-entered the game with 8 minutes left, then held Bosh scoreless in final 8 minutes after KG returned in regulation, then scoreless in overtime until meaningless FTs long after score decided
Again, KG doesn't always play that level of 1-on-1 defense. He generally saves it for when the Celts are playing a team with a dominant big man who, if removed from the equation gives the Celtics a markedly better chance to win. In those situations I guess it is deemed worth it for KG to expend the energy, be it for stretches or for quarters at a time, to be that lock-down 1-on-1 defender. So under most circumstances yeah, KG is a better help defender than a 1-on-1 guy, but he has clearly and repeatedly shown since he's been a Celtic that when facing the best and the need arises he can not just slow them down...he can take them completely off the board.