Jon Lester has been a fulltime rotation starter, posting 31 or more starts each year, for the last seven seasons. In five of those seasons, he has put together excellent work that resulted in a bWAR of 4.4 or higher. That includes his 2014 season split between the Red Sox and Athletics, where his combined performance resulted in 4.6 bWAR. That ranked seventh among all American League pitchers.
Jake Arrieta posted 5.3 bWAR for the Cubs in 2014, the best bWAR figure for a Cubs starter since Ryan Dempster's 7.0 bWAR season in 2008.
How would Lester's best seasons rate in Cubs history? Here are all the Cubs starters who have had a 4.4 bWAR year or better since 1969 -- the divisional play era:
As you can see in the table, the only Cubs pitchers who have as many 4.4 bWAR seasons (or more) than Lester over the last seven years are Rick Reuschel (six such years) and Carlos Zambrano (five). Fergie Jenkins shows up with four.
Lester's two best years by this measure -- 6.3 bWAR in 2009, 6.1 bWAR in 2008 -- would rank 11th and 12th on this list, and in general, when the Cubs have had a starting pitcher with a year that good, they have at least had a winning season and there are several playoff years near the top of that list. You have to go down to 15th place (Reuschel, 5.9 bWAR in 1973) to find a season like that in which the Cubs didn't have a winning record. Six of the 34 pitching seasons on the table above were posted by pitchers the same age (31) as Lester, or older.
Obviously, putting together a team with a winning record is much more complicated than simply having one dominant starter. But I think you can see by the list above that having a starting pitcher who can put up numbers like Lester can increases the chances of having at least a winning season, if not getting into marginal playoff contention. Presuming Arrieta can repeat his performance of 2014, the Cubs will have two such starters.
In an era where pitching appears to be more and more dominant, it looks like the Cubs have made the right move.
At 1 p.m. CT today, I'll have a similar look at the best batting seasons in modern Cubs history.