According to Andrew Baggarly, "There are plenty of reasons why this makes sense." Here are some (all?) of those reasons, with my (respectful) rebuttals in italics ...
- – Sanchez, not Cain will have to deal with Citizen’s Bank Ballpark, which is kind of a bandbox. The Phillies hit 94 homers at home this year, compared to 72 on the road ... and Cain is slightly more of a flyball pitcher than Sanchez.
Perhaps, but Cain actually gave up fewer home runs this season than Sanchez. And fewer last season, per nine innings.
— Sanchez has been flat out better the last few weeks. He was 4-1 with a 1.01 ERA in Sept/Oct, compared to Cain’s 3-1, 3.29. Sanchez also struck out 11 and gave up just two hits over 7 1/3 in a 3-2 Game 3 win in Atlanta (although Cain was very good in Game 2, as well).
It's true: Sanchez has been fantastic lately. But we're talking about just six weeks here ... and six weeks during which Cain's strikeout-to-walk ratio was even better than Sanchez's.
— Bochy’s also mentioned that he wanted to “break up the righties.”
And the advantage there is what, exactly?
— Sanchez won in Philly in August, allowing one earned and two hits over eight innings.
Here's why I don't particularly like this move ...
Sanchez is slightly more homer-prone than Cain. Not a big deal. But Sanchez throws left-handed, Cain right-handed. Something a lot of people don't realize (though I'm sure Bochy does): The Giants' home ballpark is pretty friendly to left-handed power hitters. And I would prefer to start my left-handed pitcher there, with hopes of neutralizing the Phillies' three lefty-hitting power threats.
Granted, one can make far too much of this stuff. The margins are tiny, and in fact neither Sanchez nor Cain has shown significant home/road differences during their time with the Giants.
Oh, and one more factor in the equation: Whoever starts Game 2 will have five days of rest before starting Game 6, while the Game 3 starter will have just four days' rest before Game 7. It's certainly possible that someone has determined that Sanchez will benefit more from the extra rest than Cain would. If so, I heartily endorse the decision.
Barring that, though, I don't. This looks to me like one of those small-sample-size decisions that managers make because (1) they know they're supposed to make decisions, and (2) there are enough thin, grabbable reeds for Bochy to make this one.