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Jason Bay caves, joins New York Mets

You may have heard the news that free agent outfielder Jason Bay has joined the New York Mets. After the Mets had placed an offer and waited what seemed like an eternity while Bay waited an equally long time hoping another offer would come his way, the two sides came together for a deal that seemed destined to happen (at least in my eyes).

By now, you all have heard what Jason Bay is all about. He has a traditional "old-players game" of power and patience. According to FanGraphs, over the past three calendar years, Bay has a .225 ISO and a 12.4% walk rate, 23rd and 24th respectively in each category. As a hitter, he's posted excellent numbers each of the last three seasons, compiling a .267/.362/.493 slash line that's good for a .371 wOBA.

Who is Bay replacing in the Mets outfield? Well, last season the Mets ran out a cavalcade of subpar players to man left field for them in their inaugural season in Citi Field. Leading the way was the 40-year old Gary Sheffield and his surprisingly strong .276/.372/.451 slash line. In total, eight players patrolled left field for the Mets, and they measured up to a .272/.352/.421 batting line. in other words, it was not a nightmare, but it was not the best position for them. The 2009 Mets left fielders added up to 9.5 runs above average for the season, without park adjustment. By comparison, Jason Bay brought in 34 runs above average after park adjustment last season, and CHONE has him projected at 22 runs above average after 622 PA for 2010.

While Bay is a clear improvement over the combined projections of the various players the Mets would have likely used this season, it is in the defense that Bay could run into problems. Despite the plus-plus bat, Bay has only amounted to 6.4 WAR in the last three seasons primarily because of his awful defense. According to UZR over that time span, Bay has cost his team 43 runs in terms of defense in left field, a position on the wrong side of the defensive spectrum. CHONE is optimisitc (or as optimistic as you can get), projecting him at -4 runs in the playing time 622 PA will afford him (likely around 142 games). BtB's own Steve Sommer has his own projections, and Bay comes out as a not-so-healthy -10 runs compared to average.

Let's use the average of -7 runs and tack on the average offensive value as predicted by the Fans, Bill James, and CHONE. That offense is approximately worth 26 runs above average. Adding all that up gives you a value of 33.5 runs above average, or around 3.4 WAR.

Is that worth the rumored deal, valued at either four years and $66M with a fifth year vesting option. Maybe for this season, yes. But if you expect a decline (and I've heard it said that "this skillset declines quickly with age"), perhaps of about 0.5 WAR per year, then you are looking at a deal that could quickly turn quite sour for the Mets. Any decline from this rate of production (valued at $15.3M under the 2009 FA rates and perhaps a bit high given the spending climate this offseason) and you'll begin hearing the words "sunk cost" around Bay's name. Given his age (Bay just turned 31 late in the 2009 season), I'd say those words should be coming around in two years.