Cliff Lee had one of the more amazing seasons statistically in 2010. The most glaring stat, perhaps, was his 10.28 strikeout to walk ratio--good for second all time among pitchers that qualified for the W-L title (Saberhagen bested Lee in 1994 with a whopping 11 K/BB ratio). Only 24 pitchers have managed more than one season of >=4.74 K/BB, and outside of Cy Young every one of those pitchers pitched after 1963. Of those 24 pitchers, only 13 put together back-to-back seasons with such a ratio (Lee has done it twice, but five years apart). Considering there have only been two pitchers in history to post a double-digit K/BB ratio it got me thinking: what happens the year after a historic performance? Here's a quick, back-of-the-envelope look at what Lee's statistics might look like when his K/BB ratio regresses back to its more natural state.
On average, since 2005 Lee has pitched 214 innings per season and faced about 884 batters. (Note: I removed 2007 from my calculations as it was clearly an outlier and likely to skew the results). (See here for Lee's 5-year averages since 2005, minus his 2007 season).
During that time, he's walked roughly 32 batters per season with an average K/BB ratio of 4.89. Lee has always been a low-walk pitcher, with his BB/9 improving after 2007 (as Justin's graphic showed). If Lee walks 32 batters next season that is likely to translate into six more earned runs (81 vs. 75), given that Lee's runners allowed-to-earned runs conversion rate is about 31% (due largely due to his increased ground ball rate after 2007 that has pushed down his SLG% against). Walks for Lee appear to translate to about .4 earned runs (14 more walks leading roughly 6 more earned runs), which isn't that far off from general run expectations for walks (~.3). That means his ERA would increase from 3.18 in 2010 to roughly 3.40 in 2011 (Bill James has him at 3.50). More importantly, his FIP would increase from a sterling 2.60 in 2010 to 3.20 in 2011 (again, James has him at 3.40), which would have still been good for 16th in all of baseball last year, 10th best in the NL.
The one caution here is that while Lee has been great at avoiding walks there have only been 125 instances of pitchers going in an entire season with fewer than 35 walks since 1960--Lee has two of them in the last three years. If a walk for Lee is worth about .4 runs and James is right that he gives up about 50 walks next year that translates into roughly 13 more runs for 2011, for an ERA of 3.40 and a FIP of 3.46. More human, but still solid.
Lee will still be a dominant force on the mound, particulary with the move back to the NL (9th-place hitters had a paltry .574 OPS against Lee in 2009 when he spent half the year in the senior circuit). Lee's 5-year average OPS against is .684, which is actually higher than his OPS against when pitching at Citizens Bank Park (.623 in 2009)--although the sample size is admittedly small (5 games). Even with his K/BB ratio falling back to earth and moving to a hitter-friendly park for a full year he should still be an elite hurler in the National League.