They've been giving him 6+ runs a game, that won't keep up. Webb is great, but highly dependent on the defense behind him to make double plays. If Hudson is out for a while, that could hurt Webbs win total. The D'backs give up either the most unearned runs in baseball, or close to it, so stats like that will hurt him. Getting 23 wins is an extremely difficult task for anyone (Randy Johnson only did it once, getting 24), so saying under is the safe bet.