Mr C, declining is not the word I'd use for Mussina. After 2 yrs of 4+ eras in 2004 and 2005 he posted a 3.51. That is an incline if anything.
Johnson is very frustrating. I watched almost all of his starts last season. He had 2 or 3 starts where he started using his splitter. A pitch that was in the 70s with slight bite but it was enough of a change to make him effective. He then goes to Boston, his first splitter he throws ends up on the street off Manny's bat and he never threw it again for the season. If he can develop that splitter to be used 10% of the time, he will be very effective again. But at 43, he cannot rely on an inconsistent fastball (anywhere from 91-98 last season) and a slider that bites or doesnt bite at nearly the same speed (that slider was anywhere from 80-90mph last season). He needs an off speed pitch and thus far he has refused to use it. If he uses it more often, then I'd expect him to drop that era into the low 4's. If not, he will fly past 5 on the ERA charts this season.
Pettitte is a guy who you have misjudged. His 2005 campaign was brilliant. Low 2's era and ate a ton of innings. He struggled in the first half of last season, but was brilliant again after the AS break while the stros were fighting for their playoff lives. The one thing about Pettitte is that he is not really an injury labelled guy. He will go 200+ IP and he has a yankee pedigree. He is also 34 and with a huge frame like he has and a strong work ethic, he should still be in his prime. If he comes back to NY, I'd expect 220IP 15-20 wins and an ERA right around 4-4.2. That would be enough of a bridge to get to the young kids. Right now, Sanchez, Clippard, and Hughes are very green. Bringing them up this yr would be akin to the Mutts bringing up the big 3 way too early and ruining all of them. Buying 1-2 yrs could be the difference between having 3 young kids who come up and fill out the rotation admirably or 3 guys rushed too fast and ruined. Pettitte on a short term deal fixes that last hole.