The Boston Red Sox made the offseason’s biggest move so far, finally acquiring first baseman Adrian Gonzalez from San Diego for three top prospects. With the move, Gonzalez leaves the cavernous PETCO Park for the cozier confines of Fenway Park, while Kevin Youkilis shifts across the field to reclaim his original third base position.

First, let’s look at Gonzalez’s move. In the last four years, Gonzalez has hit 90 home runs on the road. That’s an average of 23 per year. The problem for Adrian has been that PETCO Park is such a difficult home run park. At home he has averaged only 12 home runs per year. It is not unreasonable to suggest that, had Gonzalez played in a more normal park for home runs, he would have averaged closer to 45 or 46 home runs per year in those four years, rather than his actual figure of 34 long ones per season.

In The Bill James Handbook 2011, the home run park factor for all left-handed hitters playing in San Diego over the last three years is 59. That means that, overall, lefty swingers have hit 41 percent fewer home runs there than elsewhere. Gonzalez’s performance is consistent with that, though a little worse.

Also in the Handbook, we projected the lefty slugger for 33 home runs and a .890 On-base Plus Slugging mark. This is based on playing half of his games in PETCO Park. Moving him to Fenway Park, his projected batting line improves tremendously. Not only do his expected home run totals improve, but his batting average should improve dramatically as well. His home batting average over the last four years was .260 while he hit .306 on the road. In the new park, we project him to hit seven more home runs, raise his batting average by 27 points, and create about 30 more runs in the same number of at-bats in Fenway compared to PETCO.

To accommodate Gonzalez, Youkilis (an above-average first baseman) moves back to third base, replacing former Fielding Bible Award winner Adrian Beltre. While Beltre had a tremendous 2010 season at the plate and in the field, he is unlikely to reproduce the same offensive performance in 2011.

We projected Youkilis to save seven runs at first base defensively next year, compared to Gonzalez’s three. That’s pretty much a wash. At third base, Youkilis has done fine in limited playing time. There’s no reason we can’t assume he’ll continue to be at least competent at third base. But as an average third baseman, he’s a tremendous drop off from one of the best defensive third basemen in the game, Adrian Beltre, for whom we project 17 Runs Saved for 2011. All in all, that’s about 20 runs lost defensively in the Gonzalez deal for the Red Sox comparing Youkilis at first base and Beltre at third to Gonzalez at first and Youkilis at third.

However, we project Gonzalez to create 143 runs with the Red Sox next year, a full 52-run improvement over Beltre’s projected 91 Runs Created if he’d stayed with Boston.

Even if the Red Sox can’t work out a long-term extension with Gonzalez, the trade improves their 2011 squad by about 32 runs (more than three wins)! Not a bad day at the office for General Manager Theo Epstein and his crew.