While Mulder has been regressing let's give him the benefit of the doubt and assume a return to his 2005 form. Let's add to that however some normalization and removal of luck.
2005 Component Stats:
4.9 k/9, 3.1 bb/9, 0.84 hr/9, 60.5% gb%, 15.3% park adjusted hr/f%
Before I project an era/whip from those I'd like to mention that while the 15.3% park adjusted hr/f% is a bit higher than the 11-12% we would expect I think it's a symptom of the fact that he's very hittable at this point. He's not fooling batters at all which means that despite his ground ball tendancies he still gives up some home runs (though not nearly the number he would give up if he wasn't so good at inducing ground balls).
4.16 era, 1.413 whip (actual was 3.64 with a 1.38 whip)
Mulder's projected era last year would play in most fantasy leagues but isn't great. The whip on the other hand (both the projected AND actual) was a disaster. The k/9 really hurts in limited ip roto leagues or leagues that use k/9 as a stat. The ONLY thing he brought to the table was wins. With the Cardinals offense not quite the wrecking ball it's been in the past and given that a return even to last year's form is not guaranteed (he has been sliding for a few years now) I just don't see Mulder being worth holding in many formats at all. You might actually be doing yourself a favor by cutting him now even if he's only sitting in your DL slot. That way you won't be tempted to drop someone to activate him when the time comes.