The Boston Herald reports. I think SBN's Viva El Birdos might have had this before anyone. The Sox will send Duncan, who posted a .687 OPS in 260 at bats for the Cardinals this year, to Class AAA Pawtucket. Boston will pay the rest of the money ($13.5 million) on Lugo's contract.
This isn't a bad trade for either team. Lugo isn't a good player anymore, but he might have enough left to clarify a very messy Cardinals infield situation. With Jed Lowrie returning from the DL, the Sox had little use for Lugo, but the Cards can find a role for him--with Khalil Greene and Troy Glaus on the DL, they've had a bunch of mediocrities minding short and third.
As for Duncan, it's easy to forget that he posted a .952 OPS in 2006 and an .834 OPS in 2007, so there might still be a little something there. He hasn't been himself since missing time early in the 2008 season with a herniated disc in his neck. The Sox acquired Adam LaRoche today too, so it's unlikely Boston will have an immediate role for Duncan, but it's probably worth their time to take him to arbitration this offseason and spend a year seeing if he can return to his old form, or at least find some value carving out an Eric Hinske-like career as a low-average, homer-hitting bench option at first and the corner outfield spots. Duncan's unusual injury history may make his road back a tough one, but he's worth a shot.