The Mets penciled John Maine into the 2009 rotation despite uncertainty about his health, following bone spur surgery after 2008. Expecting him to return to 2007 form was a stretch but it wasn't unreasonable to think he could provide 140 innings with a ~4.30 FIP.  The various preseason projection systems pegged him around this level of performance.  Like most of the Mets, Maine's season didn't turn out quite as well as hoped.

Will Carroll of Baseball Prospectus wrote the following before the season:

Maine tried to pitch through a bone spur in his shoulder last year. The off-season surgery worked out better than that did, and he's pitching well so far. I don't expect it to be a problem once he gets comfortable, but I wouldn't be surprised by a bad April.

Carroll was somewhat prophetic, as Maine struggled mightily in April.  In fact, he wasn't very good from Opening Day through June 6, the day of his last start before hitting the DL until September with arm fatigue and various other ailments.  During this time, Maine threw 61.2 innings, striking out 42 and walking 34.  His swinging strike % during this stretch was a solid 9% but if a pitcher is going to walk that many batters he better have a K/9 higher than 6.13.

Maine didn't return until the playoff push garbage time, making 4 September starts.  He pitched better than in April-June, with a 13:4 K/BB ratio in 19.2 innings.  His last start of the season was also his best and his fastball velocity was improved from the poor early season showing.  Overall, it was a borderline replacement level season.  A decent 4.43 ERA masked a lucky .253 BABIP and 7.5% HR/FB%.  An xFIP of 5.14 and a tERA of 4.89 are better indicators of the kind of season he had.  Still, his velocity was about where it was in 2007 and an offseason of rest won't hurt his continued recovery from surgery.

Verdict: Bring him back in 2010. He's just 28 years old, is relatively cheap and his batted ball profile meshes well with Citi Field's dimensions.  Hopefully the outfield defense will be improved next season (adios Gary Sheffield, hello Matt Holliday or Mike Cameron?). It would be a mistake to guarantee a rotation spot (likewise for Oliver Perez) and his spring training performance should be under tight scrutiny.  Maine earned $2.6 million last season, avoiding arbitration.  He wouldn't cost much more than that next season.  For a pitcher who was a 2.5-3 WAR performer just 2 years ago, this is a more than fair price to pay.