If you want to know the key difference between 2010 and 2009 for Brad Penny, it wasn't returning to the National League. It was the types of pitches Penny used in the National League, and when he deployed them.
Working with Cardinals pitching coach Dave Duncan and seeing what other pitchers in St. Louis used to find success, Penny further refined pitches he already had, cut back on the usage of his four-seam fastball, dramatically increased the use of his splitter and worked at deploying the pitches in less predictable ways. The splitter was a pitch he'd had for a few years, and even dabbled with a bit. Before his time in St. Louis, however, Penny really didn't have good command of the pitch.
That changed. Dave Goold of the St. Louis Post Dispatch wrote about that during spring training last year.
"I've always been a four-seam-guy," Penny said. " … But if I get a guy to a 1-0 (count) and he's sitting four-seamer, it will help to have something with a little movement on it. We'll see if I can get this pitch there."
What Penny and Duncan seek is a secondary fastball the righthander can use to complement the heat.
"He's got to throw it to get a feel for it," Duncan said. "He's got to throw it enough that he knows what it's going to do, so he knows where to throw it to and so he can execute the pitch."
So what Penny did was rely more on his split-finger pitch -- his "Splitty" he called it in the interview. The pitch tumbles down like a sinker and has good horizontal movement. It gets swings and misses, but it also gets batters to get poor contact or drive the ball onto the ground.
Fangraphs.com stats show he threw it more than 28% of the time in 2010. The previous year, that number was about 6%. Conversely, he backed off on the usage of his traditional fastball. He used that 71% of the time in 2009 but only 46% in 2010. (Interestingly, even though it's been said Penny's not the fireballer he used to be, his average four-seamer was still 94.0 miles per hour, the fastest average of his career.)
Here are two graphs from TexasLeaguers.com to get a visual look at how the pitch works out. On these graphs, it's labeled CH for changeup, but that's because Pitch F/X had trouble getting the identification right.
This view as if you were standing on the side, the vertical drop the ball makes.
This view is if you were looking from above, the horizontal movement of the ball
The key thing about the pitch is that the release point is ideally identical to a regular fastball, so the movement of the ball is unexpected by the batter. Penny's release point was pretty much the same. The velocity came in about 6 mph slower than his four-seamer, which is a straighter, faster pitch than the splitter. The horizontal movement on the splitter was nearly twice as much.
The results were obviously some of the best in Penny's career before he was shut down. His groundball rate was 52% while the line drive rate was a career-low 17.8%. Penny also saw his rate of walks fall, resulting in the best strikeout-to-walk radio of his career (3.89). Penny finished the injury-shortened season with a FIP of less than 4.00 for the first time since 2007, when he pitched for the Dodgers. His ERA was 3.23.
Obviously we'd be able to read into these numbers with greater confidence if he'd done it for an entire year in St. Louis and not just the first two months of the season. But I think it's safe to say the lessons learned from the Cardinals organization are good ones that should help him to find more success in his second time in the American League than his first, when Penny pitched in hitter-friendly Fenway Park against a tough AL East.
By the way, you might recall this isn't the first time you've heard about a Tigers pitcher using the splitter. It's one of Jose Valverde's key weapons, and he taught it to Joel Zumaya as well. Zumaya found some nice success with it too before his injury.
So to circle back: We can't say for certain how Penny -- or anyone -- will do in Detroit this season. But I really like his chances of looking a lot more like the guy who took the mound last season than the one who didn't cut it in Boston. His growth as a pitcher is a pretty good sign. If he continues to use his splitter well, he has the potential to be a mid-level starter again if everything goes right. But he doesn't have to be. He just has to be a dependable, back of the rotation guy. I have confidence he can do that. So that's exactly the kind of addition you hope to your team makes.