Yes, seven-and-a-half. That's how many wins Johan Santana is worth over a replacement player according to Baseball Prospectus. Take a look at the PECOTA projections for Santana as a Met in 2008:
GS W-L IP H BB SO HR ERAGranted, BP defines a "replacement player" as someone extraordinarily bad and if an entire team was made up of replacement players it would project to win 20-25 games in a 162 game season. But without Santana who were the Mets other options in the final rotation spot? Based on his aggregate performance last season, Mike Pelfrey, at this point in his career, isn't much better. In fact, in 2006 with very similar WHIP, ERA, and ERA+ Pelfrey's WARP was 0.3.
Mets 32 16-8 225.0 184 60 239 25 2.94
SuperVORP WARP MORP
56.8 7.5 $22,400,000
Assuming that Livan Hernandez would be manning that spot and not Pelfrey still gives the Mets around 4.5 wins this season, as Livan's WARP has been hovering around 3 for the past three seasons.
PECOTA also has Santana's homeruns allowed returning to 2004-2006 type levels. As Nate Silver notes in the BP article, Santana was always a flyball pitcher it just seems more of the flyballs are leaving the parks at this point in his career. Either way, one would assume that pitching at Shea would help this HR problem. Well, like most assumptions, that would be wrong. Based on park factors alone, Shea is more HR-friendly than the Metrodome. It would appear - based on the 2008 PECOTA projections - that last season's increase in HR allowed was either a blip on the radar or Santana will benefit from facing less potent lineups in the National League.
One Santana should also get a boost from making half his starts on grass instead of carpet which could also reflected in the 13 less hits PECOTA projects him to allow as Met instead of a Twin. The park factors back this up for 1B and 3B, and as you would expect with its bigger gaps Shea plays better for 2B hitters.
Any way you look a it the Mets added the premier pitcher in baseball without giving up their blue chip prospect or major league talent. Unless Mulvey and Humber are anchoring that Twins rotation and Gomez is making his 2nd AS appearance in 2010 while Johan is in Port St. Lucie rehabbing his (for the second time) surgically repaired elbow, I'd go out on a limb and say the deal was a good idea.