The right-hander was 3-0 with a 4.02 ERA in 40 1/3 innings for the the Tampa Bay Rays last season. He struck out 36 while walking 27.
Nelson isn't new to Boston, having been part of the organization twice before. He spent time in the Red Sox's minor league system in 2002 after being released by the Atlanta Braves, who picked him in the fourth round of the 1996 draft. Nelson was released by Boston that same season, but he did return to split his time between the minors and majors in 2004.
Tim Kurkjian is a senior writer for ESPN The Magazine.