Last year the rotation was the most stable it has been in years, aside from the starting five only four other pitchers made starts (D.J.Carrasco, Chris Young, Chris Schwinden, Miguel Batista). That's not typical and I'm not talking about the Mets, that's league wide, you generally need at least eight starters to make it through a season.
For example a truer reflection of the necessity for pitching depth was the 2010 in which seven pitchers not in the rotation made starts (Raul Valdes, Oliver Perez, Fernando Nieve, John Maine Jennry Mejia, Pat Misch and Dillon Gee). The two years prior to that they used five and six extra starters respectively.
That's an average of nine starters per season; in addition to the five fixtures in the rotation, you generally need two more starters who log a decent amount of innings and then anywhere from 2-4 starters making an occasional spot start.
Without bringing in an established swing-man they're already relying heavily on the return of Johan Santana. Albeit the four behind Santana have been relatively durable pitchers; Dickey, Peflrey, Niese, and Gee.
We all know what happens when you put all your eggs in one basket and even if everyone remains healthy there are always those chance starts. I'll even say I'm comfortable with Schwinden as the number six starter it's after that the Mets situation gets too thin and dicey for me.
The Mets simply have not given themselves depth in the starting pitching department, they are lacking both quality and quantity. My biggest concern isn't throwing some re-tread scrub out there to fill a hole it's more about them running out of options and rushing a kid like Matt Harvey before he should be there, because they didn't sign a guy like Ross Ohlendorf.
Here's what they have behind the starting five:
D.J. Carrasco- last year he made one start lasted three innings pitched to a 7.36 ERA and overall as a reliever was horrible.
Chris Schwinden- 2011 four starts with a 4.71 ERA
Miguel Batista- Late season call up made five starts with a 2.42 ERA but at 40 y/o what can we count on from him.
Jeremy Hefner- since he has a roster spot I guess he would be next up. No major league experience and a 4.92 ERA in two AAA seasons
Chuck James- hasn't started since seven games in 2008. After injuries cost him almost three years he came back last year as a reliever.
Garrett Olson- Last two seasons when called up was used as a reliever. In all over three seasons ( 2007-2009) he made 44 starts 6.78 ERA, making 24 starts in 2008 for Baltimore with a 6.65 ERA.
That is all that is keeping Matt Harvey from Queens and in my opinion that simply isn't enough, we saw what happened when Pelfrey was rushed. Pelfrey was drafted in 2005 in pitched 96 innings in 2006 between (A+ through AAA) and then came up for four games. Harvey at least wasn't used last year but with 135 innings between (A+-AA) under his belt there is already talk of him coming up this season.