Lee hasn't been quite as effective in 2009 as he was during his Cy Young year: His walk rate is up slightly and his strikeout rate is down slightly. But he has been turning it up in July, posting a 25-to-6 K-to-BB ratio and a 2.37 ERA in five starts this month, and that was facing five AL lineups.
Lee will now face the inferior lineups of the National League. Over his career, his ERA is nearly a full run lower against the NL (3.18 as compared to 4.13 against the AL). In four interleague starts this season, he has a 2.48 ERA.
But the biggest boost to Lee's value will stem from improved run support. His run support average (4.32) was eighth lowest among AL starters this season, the biggest reason he's 7-9 despite a 3.14 ERA. The Phillies are far and away the highest scoring team in the National League, and of the other four pitchers in their rotation only
The only negative is that Citizens Bank Park is a bit more hitter-friendly than Progressive Field, but not enough to have a major effect on Lee.
He figures to make about 11 starts over the final two months of the season and should post an ERA right around 3.00. With even average run support, Lee could easily win seven of those starts.