I completely agree with Loose Cannon, I would love to sign Millsap and a defensive center (Robin Lopez or Jordan are my 2 favorites) and then try to trade up to get a solid wing player.
If you are into advanced stats look at this:
Millsap-> 4.0 Offensive Win Share, 4.3 Defensive Win Share, 1.6 OBDM, 2.6 DBDM, with a 3.7 VORP
Aldridge -> 4.8 Offensive Win Share, 3.7 Defensive Win Share, 0.3 OBDM, 0.0 DBDM, with a 1.4 VORP
What those numbers show is that Millsap and Aldridge are similar offensively (Aldridge is a better scorer but Millsap helps the offense in other ways) but Millsap is clearly the better defensive player. Pair him with a rim protector and the Celtics defense will be next level and the offense will improve with Millsap in the game.
Watch him in the playoffs and picture him on the Celts next season, he is the most realistic get for the Celtics IMO and that is great!