There is no perfect way to predict the impact of specific player transactions in the NBA; if there were, the Spurs would have patented it years ago. We all use our best estimates in grading deals and predicting the next season's standings, especially in free agency, where so many factors (including coaching changes) come into play.
Common sense can tell us a lot. For instance, the Blazers gave up a number of real good players this year. Those guys were replaced with lesser players. Hence, we would expect Portland to be much worse next season! Unfortunately, not all situations are so easily adjudicated. Consider the Mavericks, who gave up a number of players ranging from problematic (Rajon Rondo) to valuable (Tyson Chandler) and everything in between (Monta Ellis) while adding some interesting talents (Wesley Matthews, Zaza Pachulia). Or the Spurs, who added a star (LaMarcus Aldridge) and a key veteran (David West) at the cost of some solid depth (Tiago Splitter, Marco Belinelli, Cory Joseph and Aron Baynes). It's not so cut and dry to assess the impact here.
So I turn to a crude method to measure how free agency and trades impacted the eight teams who made the West playoffs last season. We'll use Win Shares. Why? It's a counting stat that is tied directly to team quality and is represented in a number easily comparable to team wins. Win Shares are not perfect, and some analysts (who will no doubt express their opinion in the comments section below) won't touch them with a 10-foot pole. But I find them to be useful in crude measures like this.
So how did the eight West playoff teams fare? (Note: we didn't include rookies because A) there are no impact rookies on these teams; B) rookies don't perform well with few exceptions; C) accurately predicting rookie production is beyond the scope of my admittedly limited expertise.)
This chart illustrates the 2014-15 Win Shares each team has lost to date via trades or free agency and the 2014-15 Win Shares each team has gained. These additions and subtractions are applied to the team's 2014-15 win total. An example! The Warriors won 67 games. They traded David Lee (3.2 Win Shares in 2014-15) for Gerald Wallace (0.1 Win Shares in 2014-15) for a (rounded) net of minus-3 Win Shares. Applied to the Warriors' 67 wins, and we come to a result of 64 expected wins in 2015-16.
Let's take another example: the Clippers. They lost a combined six Win Shares by trading Spencer Hawes and Matt Barnes and gained a combined seven Win Shares by signing Paul Pierce, Wesley Johnson and trading for Lance Stephenson. This gives them a net of plus-1 Win Share. Applied to their 56 wins in 2014-15, this method expects them to win 57 games in 2015-16. (I should note that Stephenson had negative Win Shares last season. In Indiana, he'd topped at seven Win Shares in a season. This estimate relies only on last year's data, which is another imperfection.)
The nitty gritty is ill-served by this chart -- injuries, player improvement, player regression, team schemes, minutes allotments and all of that will have bigger, unaccounted-for impacts. But I find it useful to spot interesting items for further exploration ... like, are the Grizzlies better than the Spurs?
Attention has been focused on San Antonio's two huge additions: Aldridge and West. Aldridge actually rates lower than expected in Win Shares, and West is right in line with the productive veteran range (similar to Pierce in L.A.). What isn't discussed so much is that the Spurs lost a lot of Win Shares despite re-signing Kawhi Leonard, Danny Green, Tim Duncan and Manu Ginobili. The team traded Splitter and let Belinelli walk in order to make room for Aldridge; the Spurs also let Joseph and Baynes go, decisions that seemed likely regardless of Aldridge. That's a combined 12 Win Shares out of the door. Per 2014-15 Win Shares, the additions of Aldridge and West only barely made that up.
Meanwhile, Memphis lost Kosta Koufos, who earned three Win Shares in 2014-15. They added Matt Barnes (3.5) and Brandan Wright (6.5!) in his stead. That's a net plus-7 Win Shares for the Grizzlies, and when added to their 55 wins from 2014-15 gives them 62 expected wins next season. That eclipses everyone but Golden State. Now, the Wright number looks high per our eyeballs, and the Koufos number seems low. Like I said, it's imperfect. But it's certainly something worth exploring further.
There's a big ol' elephant in the room when talking about the West playoff picture, of course: the Oklahoma City Thunder. It's pretty clear that the Blazers, Mavericks or both will be hard-pressed to return to the postseason, and the relatively status quo Pelicans might join them on the fringe. The Thunder will be back near the top given health, and the Suns and Jazz will be pressing for places, too. The Lakers, Kings and Nuggets might even make things interesting further down. Now that most of the movement is done, it's time to start figuring out how things might shake out.