There were many worrisome aspects of the loss to PHX. I am going to touch on three, in no particular order.
A. PHX's defense against Tony Parker. All Spurs fans know he has no jump-shot, however the same could not be said for most teams last year. In the playoffs it took teams two games of trying to go over the top of the pick before adjustments were made. This year both DET and PHX have gone under just about every pick and have routinely positioned their defender about 5' off Parker. When you get right down to it, the only thing that has changed about Tony's shot is the way he sets up for a free throw.
B. Tim Duncan's defense in the fourth quarter. The Spurs managed to grab a rare lead on a Horry 3P with 4:48 left in the game. On the following possession Kurt Thomas set a screen for Steve Nash. Parker went over the top as the Spurs do 95% of the time. Duncan, who was guarding the screener, sagged back towards the lane, leaving Nash with a wide open 3 which he promptly drained. Absolutely fucking pathetic defense from Tim Duncan. The only slightly acceptable explanation is that he was exhausted. Apparently he remained tired because he played the exact same defense about 60 seconds later with the exact same results. Bravo Timmeh!
C. The obvious: PHX was without Amare and they still won. I think that in general most fans overestimate the impact of one player. They look at Stoudemire's stats and see PHX missing 25+ points and 8+ rebounds when, in reality, "missing" is not an accurate description. After all, those points had to come off some shots, right? And some Sun, probably Eddie House if he's in the game, has to take those shots, no?
Now everybody let's keep our shirts on, I am not saying Kurt Thomas = Amare Stoudemire. The difference is obvious, but there are other things to consider.
Take a look at PHX's most successful lineups. Nash, Bell, Diaw, Marion and Thomas. Nash, Bell, Diaw, Jones and Marion. Nash, House, Bell, Marion and Thomas. That second lineup doesn't even have a big man and the other two have only one. And that's a big reason why the Suns are so successful. They force you to play small. If you don't they harass you on defense and try to run you out of the building. If you adapt you then are forced to play out of your comfort zone. "What's the problem?" you say! "They'll just put Amare in for Kurt Thomas and stay small."
Well then, who's Stoudemire going to guard? Duncan? Really? First off, for whatever reason Amare isn't much better than an average defender (PHX defensive EFG% was 2.4% higher when he was on the court last year). Secondly, does PHX really want to expose him to foul trouble? Of course not. There's a reason Nash guards Bowen and Rasho/Horry cover the best offensive big man. And PHX practically came out and said "We acquired Kurt Thomas to guard Tim Duncan."
So let's say PHX plays Thomas and Stoudemire, likely taking minutes away from Diaw. Now they're much less athletic. Now the Spurs don't have to go small and can clog the lane and pound the offensive glass with Rasho/Nazr and Duncan. Now they're going away from what has made them surprisingly successful this year. Now the guy leading them in +/- is on the bench.
Let's wrap this up. Presuming Stoudemire is healthy PHX will definitely pose a bigger threat to the Spurs this year in the playoffs. (Though it's worth mentioning that they got sorta stomped last year.) However his addition has a lesser impact than most people project, especially against the Spurs.
One last thing: during the WCF last year the Spurs never, EVER double-teamed Amare. They focused on stopping the open 3s and from a win/loss standpoint it obviously worked. If necessary SA will change that up this year and mix in some double teams.