Is there a premium that should be placed on having left-handed starting pitching, at the front of the rotation?
1) Unless I'm mistaken, all of the minor league talent that the Red Sox have at the starting pitching spot -- at least the guys who are heir apparents to the starting roles -- are right handed. So, let's assume if Lester leaves that we're looking at something like Lackey, Buchholz, Workman/Doubront, Webster. We still have a lefty, but the book is sort of still out on him, and he's in the 3 or 4 hole, most likely. The prospect of acquiring decent starting left-handed pitching seems unlikely.
2) The premise: Left-handed pitching pays dividends that go beyond merely the stats of the particular pitcher. Put another way, if you had two pitchers with exactly the same numbers, you'd take the lefty over the righty. This is mostly because the ability to alternative left-handed and right-handed pitching supposedly has a beneficial effect over a series -- especially over a longer series like the playoffs. This is often repeated. It could be lore. "Off-balanceness" could be a myth. Google searching does not reveal to me any studies that attempt to quantify whether teams perform worse facing a pitcher the day after facing a pitcher who throws with the opposite hand than they do with a same-handed pitcher.
3) Let's put to the side any advantage that left-handed pitchers have. To me, this is so difficult to quantify in the first place. Left-handers are better at controlling the running game, though Lester not so much. Left-handers tend to have more movement. Left-handed batter tends to be better hitters, putting a premium on having lefty pitching. (Though it's debatable whether this is the result of the platoon advantage lefties have in a league dominated by RHP, or whether positional bias plays a role -- the phenomenon that most of the premium defensive positions are not available to lefties, so they better be good hitters to make it to MLB.) But, on the other hand, left-handed pitchers are giving away the platoon advantage more often than RHP, since the majority of batters bat right.
So, let's call number 3 a wash. It really doesn't matter, because in the end you can judge a LHP by looking at his numbers. The question is whether there's an overall rotation advantage to having a guy like Lester, separate from his numbers, that will be lost and is worth paying something for? Did the Randy Johnson/Curt Schilling one-two punch make them both worth more than the sum of their parts? I'd note that in 2004, the Red Sox won the world series without, I believe, ever starting a lefty. If the top of your rotation is Schilling/Pedro, it probably doesn't make much of a difference (or, to take a modern example, I'm sure the Tigers are just fine with Verlander/Scherzer). But the 2015 Red Sox won't have that kind of punch, and it makes me wonder whether losing the Lester/Lackey tandem at the top of the line up makes Lester worth a bit more than just the loss of Lester's numbers alone for this team at this time.
