That's a possibility and some will fall into the trap of evaluating the trade based on what Manny does in L.A. vs what Bay does here. Also, some will evaluate the trade based upon whether the Sox and/or the Dodgers make the playoffs, and, if so, how far they go in the playoffs. But here's the thing, IMO:
L.A. Manny does not equal Boston Manny. The L.A. Manny is a guy who just got his way, albeit by whining his way out of town, and now needs to finish out the season doing what he does best...hit...then seek the biggest payday possible when he declares for FA.
Boston Manny is, or would have been, a guy who was bent out of shape over not getting his way, doesn't care about the team, and quite possibly would have laid down, maybe even dragging down teammates, for the remainder of the season.
The Red Sox put up with a lot from Manny, because he could hit. Many of us in these forums deplored Manny's behavior at times, but voiced that we live with it because he hits and helps the team win, that is when he's dialed into the program. Clearly, for the Sox to have chosen now to move him, things must have deteriorated tremendously, so the FO got SOMETHING for him...and that something was Bay, a guy who the Sox, maybe, can still win with.
In my estimation this trade has to be evaluated based upon an assumption...and that assumption is that, with a fairly high probability, Boston Manny would have been much less valuable over the remainder of 2008 than he'd been at any time in his Red Sox tenure, anytime in his entire career, considering all elements of his presence (i.e. his complaining, potential feigning of injury, not hustling, taking days off when a tough pitcher was facing the Sox, etc.)...that's the Manny that needs to be compared to Bay.
The FO did what they had to do.