Let's take a look at our undervalued lefty, and the legendary "left hand of God", Sandy Koufax.
As great as Koufax was, he was not an instant success, or a counted upon member of the staff, until his fourth year in the bigs, and then posted a combined losing record as a starter for the next three seasons. (27-30, with an ERA over 4.00). Meanwhile Lester became a full time starter in his 3rd season, and then produced a 50-23 record in his first three seasons. Advantage Lester.
Koufax then put a a very good season, 18-13 3.52, improved again to14-7 2.54, and then became unhittable in the next 4 seasons, going 97-27, with an ERA of about 2.00. It is those four seasons that cemented his legaccy.
Now, let's put Koufax into the modern era, and remove 25% of his starts, due to a five man rotation, he loses 38 wins from his total, down to 127 career wins against 68 losses. Put him into the AL, with it's DH, and add half a run to his ERA, 3.26. Now, lower the mound and add another 0.50 to his ERA to bring it to 3.78. Now let's look at Mr. Lester, and see his 100-56 record and his 3.76 ERA.
Not too shabby, Jon! I've always held the position that Koufax benefitted from sympathy for his injury/ailment, in other words, had his career played out backwards, that is to say, had he been brilliant first, and then fallen into mediocrity, he would be just a foot note of interest, think fernando Valenzuela. Instead, he is the gold standard for Left-handers.