It had been a quiet start to the offseason for the Red Sox, but the club made its first addition to the major league roster on Tuesday morning, acquiring right-handed reliever Tyler Thornburg from Milwaukee in exchange for corner infieldershortstopand right-hander Josh Pennington . To acquire Thornburg to work the eighth inning in front of closer Craig Kimbrel , the Red Sox gave up three solid depth pieces, but none of them have the type of upside that will likely come back to haunt the Red Sox in the long term.Red Sox president Dave Dombrowski had listed an eighth-inning reliever as a priority this offseason and he appears to have gotten his man, as Thornburg will slide in as one of the top pitchers in the Boston bullpen. Once a notable starting pitching prospect for the Brewers, rated as the 100th prospect in baseball by Baseball Prospectus before the 2013 season, Thornburg broke out in a big way in 2016. In 67 innings for Milwaukee, the 28-year-old Thornburg pitched to a 2.15 ERA with 90 strikeouts and 25 walks and was worth 2.0 fWAR. Adding to his value, Thornburg is arbitration-eligible for the first time this offseason and will not hit free agency until after the 2019 season. However, he does carry a bit of an injury history, suffering an elbow injury in 2014 that nearly led to Tommy John surgery, but he instead opted to receive a platelet-rich plasma injection, which helped him avoid going under the knife.-----