There's an interesting fan poll on ESPN today as to who is the most important starting pitcher to the AL East pennant race: Clemens, Lowe, Martinez, or Mussina. Martinez is the run-away leader if you're interested.
However, a better question may be who will be the most important relief pitcher. The Yankees currently have three of the top five relief pitchers in the AL in innings pitched. Mike Stanton, Ramiro Mendoza, and Steve Karsay are all over 50 innings. Given that we are only halfway through the year, the possibility that some or all of them could pitch over 100 innings is very real (though only Mendoza projects to over 100 right now). I can't remember the last team that had three relievers pitch over 100 innings each (I'll see if I can find out). It is especially surprising given the Yankees veteran starting pitching core.
With the acquisition of Jeff Weaver and the imminent move of Sterling Hitchcock to the bullpen once he returns from the disabled list, this may alter the Yankees use (or over-use) of certain members of their bullpen. But with both Hitchcock and Clemens on the DL, this solution may not be as easy to implement as one would expect. Also, given the suddenly vincible Mariano Rivera and the possibility of an injury or a mechanical problem as the root cause of this problem, one or more of the middle relievers may be pressed into closer duty, thereby putting more of a strain on the middle relief corps.
This is something to be watched depending on how certain scenarios play out (injuries to and effectiveness of the players involved) going forward. I wouldn't be surprised if the Yankees picked up a veteran reliever for some depth down the line.