Hitting in the bottom third of the lineup over the span of June, Ellsbury is hitting .328 (21 for 64), with a .413 OBP and a .500 SLG.
Hitting leadoff, Pedroia has fallen into a well-publicized slump, hitting .207 (18 for 87), with only a .266 OBP. Those are a drastic fall from the April and May numbers.
Add onto that their respective numbers with runners in scoring position. This is, in fact, one of the few areas that Pedey has not seen a big-time drop-off. His season RISP number is still an impressive .357, whereas Ellsbury has been unable, despite his success over the last month, to crank his RISP up over .273. Respectable, but not overwhelming. Even where they flip the good for the bad, the numbers suggest that they're in their placement in the lineup is off.
Even the theory that Ellsbury's speed might be better utilized low in the lineup, where he can feel free to run without destroying situations for the 2-5 big dogs, has failed to pan out. Ellsbury's steal numbers, despite the increased on-base percentage, are exactly even with his numbers out of the lead-off spo, if not a little lower.
In fact, there's really only one stat that seems to offer empirical proof in favor of the move. 17-6. That's the Sox record over the month of June. Talk about playing a trump card.
But as Pedey continues to struggle (though come on, don't you love him even more when you see him making something out of nothing, desperate to get out of this slump, turning an infield hopper into a double, drawing balks, scoring on passed balls?), and Ellsbury continues to climb, it's going to be harder and harder for Francona to brush it off and say why mess with a good thing. It's a long season, and it's all about making the right adjustments at the right time.
Update (4:35 pm): According to the Fox broadcasting team, Terry Francona told them that he plans on keeping Pedroia and Ellsbury in their current lineup spots for the remainder of the season. Big surprise. Not sure if this information was accurately relayed, as the the guys definitely didn't seem to think it much of a story. Wonder if there'll be any follow up on that in the post-game.