This is a REAL tough one. It really depends upon how his team looks on opening day. Sure, run support matters (ie will Sexson be back? can we add a bat?) but IMO that's not the biggest factor in determining Webb's effectiveness. Webb relies very heavily on his infield and AZ used a bunch of "baby backs" who defensively (and for the most part offensively) should've still been learning in AAA or even AA. Will these kids make progress? Will we add a decent major league infielder or two? To me, those are the biggies in answering the question.
I haven't looked at the numbers, but Webb himself threw pretty well from about July on. I'd guess most of the damage to his BB/9 ratio was done early on when he pressed too hard for strikeouts (putrid defense had him rattled IMO) His bullpen should be adequate enough to preserve most of his leads, as well.
But HOW MANY WINS??? Good question at this point.