from Boston.com:
In adding LaRoche, the Red Sox will have to alter their major league roster, meaning a Red Sox position player will likely be optioned or placed on the disabled list whenever LaRoche arrives. This is purely speculative, but Mark Kotsay has been battling a calf injury, and while it improved over the All-Star break, the Sox could give him more rest without sacrificing any infield versatility by putting him on the DL.