At first glance, you'd think that he's just been flat out bad. But his numbers may tell a different story.
Now when Marcum's on, he's pretty good. On May 26 against the Braves, he was stellar (7 IP, 2 R, 4 H, 12 K, 0 BB). Of course, the offense didn't produce, so naturally he left trailing 2-1 in a game he should have won. Then there was the 20-inning game against the Marlins (1 R, 5 H, 7 K, again 0 BB)--and again the Mets couldn't get it done with runners on (1st and 2nd w/ 1 out in the 13th, again with 2 out in the 14th, runner on 2nd in the 15th, etc.) as they lost 2-1.
Here's another set of numbers for you. Marcum has shown remarkable control over his outings. In 59.1 innings, he's allowed just 15 walks (four last night). That's just over 2.28 BB/9. His WHIP over that time is 1.336. The only problem is (1) he hasn't shown the consistent ability to go deep into games (his IP tells the story, plus four starts with 4.2 IP or less) and (2) the run support issue.
I came across this article today that dives into some more advanced stats and why it probably would be a good idea for the Mets to remove him from the rotation. If so, that would leave the rotation at:
Gee (who has come on very strongly in his past four and should be 6-6, but again, run support)
Niese (got shelled in back-to-back outings against Atlanta and Pittsburgh in May but has pitched well of late. Should have won the Yankees game, but...)
Hefner (seven of his last 10 starts have been quality starts. Out of those seven, four have seen him allow 2 ER or less)
Now, with Wheeler coming into the picture, there's a chance the team could keep Marcum as a starter and go to a six-man rotation, or, if they want to save money, put him in the bullpen at least short term. That way they have a fall-back option if Wheeler struggles and the team is forced to send him back to Las Vegas.