When the Mets acquired John Maine, he was a relatively unknown player who had pitched a total of 43.6 innings of 6.60 ERA baseball with 5.6 BB/9 and only 5.2 K/9 in two brief stints with the Orioles. Once he had his shot with the Mets, though, Maine was generally impressive. From 2006-2008, he pitched relatively well, with a cumulative 109 ERA+ in those seasons. He was by no means an ace, but he certainly pitched well enough to be a member of a competitive major league rotation.
Since 2008, Maine has only made 24 starts and pitched only 121.0 innings, and heading into the winter, he's eligible for arbitration, meaning soon-to-be-official Mets GM Sandy Alderson will have to decide whether or not to tender him a contract. In addition to the possibility that Maine might miss 2011 entirely because of his shoulder surgery, there are a few reasons that Alderson should not tender him a contract this winter.
Maine's walks per nine innings have steadily increased since he joined the Mets in 2006 while his strikeouts per nine have been good but not great. 2009 and 2010 are small samples, but since the beginning of 2008, Maine has not struck out enough hitters to compensate for all those that he walked. This is not a good trend.
With those peripheral stats, it should comes as no surprise that Maine's ERA, FIP and xFIP have all been generally on the rise since he joined the Mets in 2006.
In addition, the velocity on Maine's pitches dropped off in a big way the past two seasons, with the alarming drop in his fastball velocity coming in 2010. Every time Maine took the mound this season, it seemed impossible that he could sustain success with a fastball sitting in the 88 mph range. It's entirely reasonable to chalk up the decline in Maine's velocity to his shoulder injury, but there is no guarantee he'll ever regain his former velocity in the future.
Finally, the difference between Maine's fastball and changeup dropped from 7.8 mph to 5.8 mph. While the velocity of his fastball slowed down suddenly, the velocity of his changeup declined gradually. Although 2 mph may not sound like much, the less the difference between these two pitches, the less likely they'll be to fool opposing hitters.
Combine the stats above with the fact that Maine made $3.3 million and underwent shoulder surgery in 2010, and he seems like an obvious non-tender. It's not impossible that Maine could one day return to being a productive pitcher, and if the Mets wanted to bring him back on a minor league deal with a spring training invite for 2011, there would be little harm in doing so. Keeping him around for millions of dollars, though, would be foolish at best.