For starters, we'll assume that the defense from both Placido Polanco and Brandon Inge will remain constant – while it's hard to predict them going one way or the other, neither has shown any noticeable decline (and Polanco may well be coming off a career year).
Instead, this will focus solely on the changes around the diamond, that being replacing Sean Casey at first base with Carlos Guillen, and replacing Guillen at short with Edgar Renteria. The thought being of course that Renteria is a clear upgrade at short, and Guillen would be a clear upgrade at first, vastly improving a defense and in turn benefiting the pitching staff.
But do the numbers bear that out?
Let's start at first base.
In 2007, Sean Casey had a zone rating of .886 (zone rating being defined as the percentage of balls hit into a player's defensive zone). That zone rating ranked Casey fifth among all eligible first basemen, and well above the .845 average at the position.
In addition, Casey's fielding percentage (percentage of putouts out of total opportunities) ranked fourth among all eligible first basemen at .998. So, it's safe to say that even though Casey had aged, he was still one of the best defensive first basemen in baseball.
Conversely, Guillen's numbers were extremely limited, as he only saw time at first in 36 games (and totaled less than 180 innings of time there). Guillen actually had a perfect zone rating of 1.000, however, that is likely only due to his limited time at the spot. It's not unrealistic to assume however that given's Guillen athletic ability; he would be an upgrade in the percentage of balls he could get to. His fielding percentage likewise would remain strong.
So, from the view of the first base spot, the Tigers already we're receiving above average defense, so while Guillen will likely be a slight upgrade, we're talking about an upgrade on an already plus position.
Meanwhile, over at shortstop, the main cause of this move was the sentiment that Guillen's defense was no longer adequate at shortstop. This case isn't really debatable.
Since becoming a Tiger, Guillen's zone rating has fallen each year, going from .837 in 2004, to .807 in '07. Guillen's error total consistently rose, and his fielding percentage consistently fell. In 2004, he committed 17 errors with a .974 fielding percentage. In '07, he committed 24 with a .955 fielding percentage.
How do those '07 numbers rank amongst all shortstops? Simply put, not good. His zone rating was well below average (.822), and his fielding percentage was actually the worst in all of Major League Baseball among qualified shortstops. Maybe Guillen would have been able to hold up for another year or two at short, but he clearly wasn't going to be an advantage for the Tigers there.
Summarized quickly, not only was Guillen getting to far less balls than he used to (or than he should according to the position averages), but he was struggling to convert those opportunities into outs.
Insert Edgar Renteria. The hope was that adding a two-time Gold Glove shortstop to the mix would give the Tigers a significant upgrade at shortstop as well. While Renteria was at one time a Gold Glover, he no longer puts up the numbers that indicates his defense is at that level.
In fact, Renteria has seen his zone rating fall from .871 in 2003 (his last Gold Glove year) to an even .800 in '07, a number actually lower than Guillen's. While Guillen's ailing knees were a large concern, it's unlikely that the Tigers will see a significant (if any) improvement there.
Where Renteria is an upgrade however is in the fielding percentage department. In just a handful more opportunities than Guillen, Renteria committed less than half of the errors that Guillen did (11 to 24). His fielding percentage left him somewhat above average (.977 to .972), meaning that while he might not get to any more balls than Guillen would have, he's much more likely to convert the ones he gets to.
The Tigers made these moves as all around upgrades, hoping to improve the team both at the plate and around the diamond. Given the above, the Tigers are likely to see limited gains both in balls reached at first, and in conversions of balls reached at shortstop.
This should help give the Tigers a slight bump in their overall defensive ability, and also make things a bit easier on the pitching staff. But those hoping for an iron wall to be put around the infield, keeping every ground ball from getting by, are likely going to be disappointed with the '08 version of the Tigers' infield.