Executive Vice President/General Manager Ben Cherington made the announcement.
Machi was claimed off waivers from the San Francisco Giants on Tuesday. The 33-year-old had been designated for assignment on July 20. In 33 outings for the Giants this season, all in relief, he went 1-0 with a 5.14 ERA (20 ER/35.0 IP) and 22 strikeouts, making 23 scoreless appearances. Since returning from a stint on the 15-day disabled list with a left groin strain on June 23, he has given up four runs in 8.2 innings (4.15 ERA). This season, he has held left-handed batters to a .120 batting average (6-for-50) and just one extra-base hit. In his big league career, all since 2012 with the Giants, Machi has gone 11-2 with two saves and a 3.24 ERA (58 ER/161.0 IP) and 128 strikeouts. In 122 appearances between 2013 and 2014, the first two full seasons of his major league career, the native of Venezuela ranked 10th among National League relievers (min. 100.0 IP) with a combined 2.49 ERA (33 ER/119.1 IP). He was a member of last year's World Series Champion Giants team, making seven appearances with no record while allowing five runs in 5.2 postseason innings.
Aro, 24, returns for his second career major league stint, both this year. He has appeared in three games for the Red Sox, debuting on June 25 against Baltimore. Over his three relief appearances, he had an 11.57 ERA (6 ER/4.2 IP) with three strikeouts. In 25 minor league appearances between Double-A Portland and Pawtucket this season, the right-hander is 3-3 with one save, a 2.95 ERA (18 ER), and 60 strikeouts in 55.0 innings, holding opponents to a .205 batting average (42-for-205) while allowing just 16 walks. In 17 games since being promoted to the PawSox on May 14, the Dominican native has totaled 41 strikeouts and just eight walks in 32.2 innings, posting a 3.03 ERA (11 ER). Signed by Boston as an international free agent in June 2011, Aro has gone 16-15 with 12 saves, a 2.80 ERA (88 ER/282.2 IP), and 279 strikeouts in 92 outings (12 starts) over parts of five minor league seasons in the Red Sox system. Prior to this season, he had never pitched above Single-A, spending most of 2014 with Greenville, where he was named a mid-season South Atlantic League All-Star.
Nava, 32, played in 29 games for the Red Sox this season, going 10-for-66 (.152) with two doubles, seven homers, eight walks, and six runs scored. He appeared in 15 games in right field (12 starts), seven in left field (one start), and six at first base (five starts). The native of California spent from May 26-July 20 on the disabled list with a left thumb sprain. In a 10-game rehab assignment with Triple-A Pawtucket, he hit .250 (9-for-36) with four runs, a double, a home run, eight RBI, and two stolen bases. Signed by the Red Sox as a minor league free agent in 2008, Nava's major league career spans five seasons, all with Boston (2010, 2012-15). He has hit .267 (351-for-1,315) with 87 doubles, one triple, 23 home runs, 169 RBI, and 148 walks in 424 career big league games.
Layne, 30, has appeared in a career-high 40 games for the Red Sox this season, going 0-1 with one save and a 3.51 ERA (13 ER/33.1 IP). For his major league career, Layne has posted a 4-4 record and 2.67 ERA (23 ER/77.2 IP) with 77 strikeouts in 110 outings, all in relief, between the San Diego Padres (2012-13) and Red Sox (2014-15). Originally selected by the Diamondbacks in the 2007 June Draft, the St. Louis, Missouri native is 45-47 with 15 saves and a 4.28 ERA (360 ER/757.0 IP) in 279 minor league games (94 starts).