He's limited to 1B and not a defensive asset there. He doesn't have elite power. His upside is basically Billy Butler, right? Ellsbury has been an above average major league regular. Alonso has a shot at being an above average major league regular.
That sort of a trade lacks upside for the Red Sox. I could see that sort of a deal happening mid-season for a team trying to capitalize on an arb eligible guy before he gets expensive. But if you're the Sox, aren't you still gunning for a real impact guy at 1B, namely Adrian Gonzalez or Prince Fielder? And isn't Ellsbury one of the, if not the, key chits there? I just don't see this making sense from the Sox perspective in light of the alternatives.