KG is a better player than DJ... Despite that, DeAndre Jordan is a better asset than KG for 3 reasons:
1) The quantity of teams that are suitors for DJ's services are more than KG's
2) The value of Jordan is superior to Garnett's on the trade market
3) The direction the front office has chosen for the Celtics
KG has a limited amount of suitors... Lottery teams won't trade their assets for KG because it wouldn't fit into their long term plans and it's likely that he would reject that option even if they were interested in his services. Playoff teams that would actually want KG, usually aren't willing to give up one of their core pieces for a veteran that has 0-2 season(s) left (this being one of the rare cases... and as an example of what offers teams receive for these veterans: see Suns sending Steve Nash to Lakers trade). What makes this even clearer is KG's no-trade clause and the opinion that KG will not waive it to a Doc Rivers-less team.
People are looking at his offense, defense, poor FT shooting, and salary, and comparing it to KG; GM's look at his age (25), size, athletic ability, the fact that he's averaged 8 PPG, 8 RPG, 2 BPG at one point or another, and the single most important factor: potential. An example of this is Jordan's offer from the GS Warriors and the subsequent matching of the offer by the Clippers. Not 1, but 2 different NBA franchises found Jordan valuable enough to pay him $43M over 4 years.
Just taking look at a few rosters... here's a brief list of what Jordan, straight up, can feasibly can bring in return:
Hawks - S&T Josh Smith
Bobcats - Gerald Henderson and/or 1st
Cavs - 1st or Varejao
Mavericks - Brandan Wright and 1st
Rockets - 2013 Draft Picks or Asik
Pelicans - Eric Gordon
So even if Jordan isn't seen as the long term option of the organization, this is what his value could be estimated and there are more feasible combinations with packaging.
The 3rd reason speaks for itself, if you can see the writing on the wall.
I think the Celtics roster as currently constructed is a contender (the caveat being that players are healthy enough to play) so I wouldn't have a problem running it back, but the situation being as it is (Doc intent on leaving to LA and taking KG and Pierce with him), I think that getting Deandre Jordan (and 2 1st round picks) are good commodities to start with. He might even turn out to be a Tyson Chandler/Roy Hibbert level player.