The San Diego Padres finally pulled the trigger on a move that has been anticipated for a long time, trading first basemen Adrian Gonzalez to the Boston Red Sox for four prospects (RHP Casey Kelley, 1B Anthony Rizzo, OF Reymond Fuentes, and a PTBNL).

Trading Gonzalez is immensely difficult, of course, because he has been the face of the franchise for the five seasons he’s played in San Diego, transforming from a very good player in his first three seasons to an MVP candidate in his last two.  You probably know the numbers by heart, but for one final time, courtesy of Baseball Reference:

Over those five seasons, Gonzalez didn’t even earn $10 million, so it is understandable that he wants to cash in when his current contract expires after the 2011 season. While it would be great if Gonzalez could have been retained beyond next season, it is simply unrealistic due to the financial issues surrounding the Padres (both the ownership issue and the San Diego market in general).

The only real question remaining was whether or not to deal Gonzalez now, wait until the trade deadline, or hold onto him for one more season and accept some draft picks in return. The Padres apparently did their analysis, and concluded that trading him now would best suit the organization.

The next issue, then, is how much surplus value Gonzalez has. Assuming that he’s a 5 WAR player and that teams will pay $4.5M for a marginal win on the free agent market, Gonzalez should be worth around $22.5M next season (if paid as a free agent). That number could easily be higher or lower depending on how you evaluate Gonzalez and what figure you use for $/win. It is merely a reasonable estimate.

Gonzalez will earn just $6.3M in 2011, therefore his surplus value ($22.5 minus $6.3) is around $16.2M. Factoring in the $5M estimate for the draft picks the Padres would have received when Adrian left for free agency, his total surplus value end up at $21M.

While that’s a nice number, it’s limited by the fact that Adrian only has one year left on his (extremely reasonable) contract. A player with, say, two or three years left before free agency, despite being less talented than Gonzalez, may garner more in a trade simply because they have more surplus value.

Next question, please. How much are the four prospects worth that the Padres acquired? For that, we can use the estimated values developed by Victor Wang, and displayed in a chart on this post.

The most highly regarded prospect the Padres will receive is right-handed starter Casey Kelley. Kelley is a 21 year old righty who didn’t decide to pitch full time until late 2009 (he was a great shortstop in college). Kelley sits in the low-to-mid nineties and has two solid off-speed offerings in the curve and changeup.

He struggled a bit last season, but it’s tough to get too caught up in 95 innings of minor league work, especially considering that he was 20 years old and in double-A (not to mention, the aforementioned switch to being solely a pitcher should pay off in the future). According to Baseball America, Kelley ranked as the 24th best prospect in baseball prior to last season – worth ~$16M.

The Padres have also acquired 21 year old first basemen Anthony Rizzo. The left-hand hitting Rizzo has put up solid, if not spectacular minor league numbers, and is also reportedly a very good fielder. Rizzo hit .260/.334/.480 last season, splitting time between high-A and double-A ball. Ranked as a B- prospect by John Sickels (last season), we  can estimate his surplus value at $5.5M.

Reymond Fuentes is the youngest prospect heading San Diego’s way at just 19 years old. Again, his numbers aren’t overly impressive, but he’s a young, toolsy outfielder who is certainly going to take some time to develop. Also rated as a B- prospect, we can estimate him at $5.5M.

In conclusion, here are our very rudimentary estimations:

Frankly, I am *not* confident in these numbers, for numerous reasons. They do, however, give us a simplified model to show how a deal like this may be viewed from each side. The Red Sox get the most valuable player, a guy they can build around for years – if they lock him up long-term (which is very probable). The Padres cash in their most valuable chip and receive three (maybe four) very nice prospects to hopefully build around in the future.

While a lot of people wanted the Padres to try to compete in 2011, it seemed to me that would be unlikely even with Gonzalez. Last season they caught lighting in a bottle and played great, but the chances of doing that again are just not that likely. In the end, it was time to trade Gonzalez, at least from a baseball perspective.

As Marc Normandin nicely sums up on his Red Sox blog, Boston wins in the short-term by getting Gonzalez in the arms race that is the AL East and the Padres win long-term by acquiring some nice pieces for the future.

The San Diego Padres have finally cashed in their most valuable asset, trading Adrian Gonzalez to the Boston Red Sox for three prospects (RHP Casey Kelley, 1B Anthony Rizzo, and OF Reymond Fuentes) and a player to be named later.

The Padres are clearly going to have a tough time competing this season, dealing away by far their best offensive force. However, they’ve just made an already solid farm system that much better, adding three highly regarded prospects into the mix.

We’ll have more analysis as the day unfolds and things become official. For now, feel free to discuss the trade here.