Because the Blazers had no one else playing well.
Do you consider the players to have any responsibility, or is all player performance due to the coach? What "counters" can you make when no one else is capable of making plays? The Blazers/McMillan definitely tried exploiting Nash defending Batum, but Batum was also playing hurt in that series. Aldridge was given touches, but he wasn't aggressive (as per his career until last season) and didn't force the Suns to adjust their defense.
You're taking the easy way out, analytically, when you say "They didn't do well, the coach should have done SOMETHING." He did try things...at what point does being less talented (with Roy a non-entity and Batum hurt) matter? And is there some specific counter to Gentry putting his best defender on the one Blazer playing well that you think McMillan should have utilized?
One bad game doesn't tell us anything. If someone used one good game (like, say, the Blazers beating the Suns, on the road, without their best player) to illustrate that McMillan was a great coach, you'd suddenly see the value in using larger sample sizes than one game.