This 31 year old journeyman is on his 5th major league team in his 8th big league season, and the first with the Giants. He became the Giants 5th starter by default when Giants prospect Madison Bumgarner struggled in spring training and lost his velocity and effectiveness. He is more or less a placeholder until if/when Bumgarner is ready (he's 20 years old, but looks to be regaining his velocity in his last 3 minor league starts).
When Wellemeyer made his major league debut with the Cubs in 2003, he was throwing 94 mph; in 2010 his average fastball is 89.6. He's experimenting with a cutter last year and a little this year, while featuring a slider (82 mph), a curve-ball and a change up (both of which he will only throw 5-6 times a game). His fastball hasn't been an effective pitch since 2004, and he has relied on his slider for most of his limited success. For his career, he strikes out 6.6 K/9 (below league average), and walks 4.4 /9 (above league average), for a career ERA of 4.78. Not bad for a 5th starter, but he won't win many games with this below-average offense behind him.
For the year he is 1-3, with a 5.55 ERA and a 1.48 WHIP, but most of that damage came in his first two starts of the year (6 1/3 innings, 4 ER, and 4 innings, 7 ER). His last appearance was three innings of scoreless relief. He's a fly-ball pitcher against whom the Mets should have success.