Boston has acquired the 26-year-old in exchange for a player to be named later or cash. Outfielder Ryan Kalish has been placed on the 60-day DL to make room on the 40-fish for Carp.
The Red Sox were believed to be competing with a few clubs for Carp -- including the Twins and Astros -- which could have complicated Boston's pursuit of the potential platoon player. Both clubs finished with a worse record than the Red Sox last season, so if either had actually put in a waiver claim for him then Boston's chances of making a trade with Seattle would have been shot.
Carp, 26, had a not-so-great injury-plauged season for the Mariners in 2012, but is a solid .255/.327/.413 hitter with 18 home runs in 608 career plate appearances over four big-league seasons. Carp actually has a bit of a reverse platoon split in his time with the Mariners, but small sample size issues abound there.
The left-handed hitter is a prime candidate to platoon with Mike Napoli at first base and Jonny Gomes in left for the Sox, so it's now just a matter of whether he can put on a better show in camp that non-roster invitee Lyle Overbay.