The announcements were made by Executive Vice President/General Manager Ben Cherington.
The 29-year-old Nava will be in his first Major League stint of the season. He has played in 27 games with Pawtucket in 2012, batting .316 (30-for-95) with seven doubles, a triple, three home runs, 17 RBI, 19 runs, 14 walks and a stolen base. Nava has hit at a .328 clip (22-for-67) off right-handers and is batting .452 (14-for-31) with nine RBI in eight May contests. With the PawSox, he has primarily played left field (16 games), but also has appearances in right field (2 games) and at designated hitter (9 games). Signed by the Red Sox as a minor league free agent in January of 2008, Nava has appeared in 60 career Major League games, all with the Red Sox in 2010, and tallied a .242 average (39-for-161) with 14 doubles, a triple, a home run, 26 RBI, 23 runs, 19 walks and a stolen base. The switch-hitter made his Big League debut against the Phillies on June 12, 2010 and became the second player ever to connect for a grand slam on his first pitch seen in the Major Leagues.
Mortensen, 27, was recalled from Pawtucket on May 2 and has posted a 0.96 ERA (1 ER/9.1 IP) with 12 strikeouts and no walks in three relief appearances for the Red Sox. In six games with the PawSox to begin the year, he was 2-2 with one save, a 0.90 ERA (1 ER/10.0 IP) and 10 strikeouts. Acquired by Boston from Colorado in January, the right-hander has appeared in 27 Major League games (13 starts) with the Cardinals (2009), Athletics (2009-10), Rockies (2011) and Red Sox (2012), going 4-8 with a 4.74 ERA (55 ER/104.1 IP) and 62 strikeouts, including a 3.00 ERA (12 ER/36.0 IP) in 14 Big League relief appearances.
The 28-year-old Thomas made seven relief appearances for the Red Sox to begin the season, allowing four runs in 4.2 innings (7.71 ERA) with four strikeouts and two walks before an April 26 option to Triple-A Pawtucket. In four outings since joining the PawSox, he has posted a 3.38 ERA (2 ER/5.1 IP) with five strikeouts compared to no walks. A fourth-round selection by Seattle in the 2005 First-Year Player Draft, Thomas has made 27 Major League appearances out of the bullpen between the Mariners (2008), Pirates (2010) and Red Sox (2012), going 0-2 with a 6.65 ERA (16 ER/21.2 IP) and 11 strikeouts.