It's an interesting question, but one which is difficult, if not impossible, to answer. But, hey: we specialize in "impossible" here at the SnakePit. ;-) So let's take a stab at looking at the numbers posted by Hudson and Brandon Webb, to see if we can project anything going forward.
The first problem is, Webb didn't reach the majors until a couple of weeks short of his 24th birthday. Hudson made his debut. while with the White Sox, at age 22. However, once Webb reached the majors, he was basically there for life. Hudson spent the vast bulk of 2009, his first season, in the minors - and, to make matters even more complex, appeared at four different levels, A-ball, High-A, Double-A and Triple-A, in addition to the majors. And the following year, his time was divided almost equally between Triple-A (17 starts) and the majors (14 starts). Oh, and he was traded mid-season, going from Chicago to Arizona. Pick the sabermetric bones out of all that.
I'm not even going to bother. Instead, I'm going to focus on Hudson's time in Arizona, as he has now made a total of 31 starts, close to a full season. I'll compare those numbers against Webb's first 31 starts in the majors, because most of those took place over approximately the same period. Webb had 28 starts through the 2003 season, at the end of which, he was approximately the same age as Hudson is now (24 years, 4 months). Here are the two men's respective lines for the time concerned:
Wow. I genuinely did not know the ERAs would end up being basically identical - I hadn't looked at Webb's numbers when I picked the data sample. But it seems to show that he and Hudson have been almost as effective over the matching periods of their careers with us. Hudson has gone deeper into games, but has allowed more hits; Webb has struck out more batters, but has also given out a good number more free passes, so has the higher WHIP. What is surprising, is that the home-run rate was not enormously different: 0.55 per nine innings for Webb, 0.59 for Hudson. Given Webb's sinker and Hudson's fly-ball tendencies, I'd have expected a bigger gap.
If we look at a defense-independent number such as FIP, using the basic formula given here [the link is also a nice explanation of FIP, for those not familiar with that it means], Webb's stats come out at a 3.27 ERA, and Hudson at 3.02, so our current ace would appear to have the edge. Whether that can be sustained going forward would appear to depend on a couple of things. Firstly, Hudson's HR rate has been lower than you'd expect, this season in particular. In 2011, only 3.6% of fly-balls he has allowed have left the park, less than half the average MLB rate (7.4%). That number will probably regress towards the mean.
However, one way to combat that is simply by allowing fewer fly-balls: at the risk of stating the bleedin' obvious, ground-balls don't often result in home-runs. And there is some evidence to suggest that's what Hudson has been doing. The MLB average ratio for GB/FB is 0.80, and in 2009 and 2010, Hudson skewed heavily towards fly-balls, with ratios of 0.50 and 0.59. This year that number has increased to 0.74; while still fly-ball skewed, it's much closer to the mean, and that should help, especially in a hitter- and home-run friendly park like Chase. So let's take a look at the possible reasons for that.
The main change in Hudson's armory this year has been the rise of his slider, which was a focus of his development in spring training this season, in addition to his fastball and change-up. While he is still throwing the change-up around 23% of the time, the slider is seeing much more use, up from 10.5% to 16.6% of pitches by Hudson. That doesn't seem to have result in additional swings and misses - batters are actually making more contact against Hudson (on 78.4% of swings, compared to 75.0% last year). But there is a lot more contact out of the zone: 68.7% of the time, up over 11% on 2010.
It's no stretch to think, that it's hard to do much with a pitch, when you swing at a ball rather than a strike. This was noted by Nick Piecoro today, who says Hudson's slider, "has allowed him to throw the ball toward the bottom of the strike zone... Those balls have turned into easy outs with help from the defense behind him." It is, to some extent, a work in progress - just ask Shaun Marcum, who ripped an 0-2 hanging slider into the bleachers for a pitcher's grand-slam earlier this month. But after a shaky April, Dan has been nails: Hudson hasn't lost since May 12, and in the last fourteen starts, is 8-1 with a 2.89 ERA.
It's worth noting that it took Webb a couple of years to become a bona-fide ace. While his sophomore and third seasons were solid, he posted ERA+ of 129 and 126 respectively - very good, rather than great. It wasn't until 2006, his age 27 season, that he broke out, winning the Cy Young with an ERA+ of 152. That would put Hudson on pace for 2014, and he (along with Ian Kennedy) are both fully under Diamondbacks control through the end of the season after that. If our pitching prospects pan out - and I appreciate that is an if - the Diamondbacks' rotation by that point could be absolutely stellar.
So, in conclusion. Though Hudson and Webb are entirely different pitchers, their pure results to date with Arizona are surprisingly close. Dan certainly has grown appreciably as a pitcher, since coming over from Chicago, and has the potential to develop into a front of the rotation starter for the D-backs, one that can carry them through the middle of the decade. With David Holmberg ripping up the minors (he turns 20 tomorrow, and has a 2.58 ERA in 17 starts at A and High-A, plus a K:BB ratio of 99:19), the trade is looking pretty damn good. Jerry Dipoto may only have been GM for less than three months, but his legacy could be pitching platinum.