Executive Vice President/General Manager Ben Cherington made the announcement.
Layne, 30, returns to Boston for his third major league stint of the season. In 40 games with the Red Sox this year, he is 0-1 with one save and a 3.51 ERA (13 ER/33.1 IP) with 32 strikeouts. The left-hander was optioned to Pawtucket on July 30, a day after his last big league outing against the Chicago White Sox, and has since made three appearances for the PawSox, allowing three runs in 2.2 innings. He has three saves and a 3.52 ERA (3 ER/7.2 IP) in seven Triple-A outings overall this season. Layne has 110 games of major league experience between the Red Sox (2014-15) and San Diego Padres (2012-13), all out of the bullpen, and has combined to go 4-4 with three saves and a 2.67 ERA (23 ER/77.2 IP) in his career.
Masterson, 30, pitched in 18 games this season for the Red Sox, going 4-2 with a 5.61 ERA (37 ER/59.1 IP), 27 walks and 49 strikeouts. His 18 appearances were evenly split between Boston's starting rotation, where he was 3-2 with a 6.14 ERA (30 ER/44.0 IP) and 32 strikeouts in nine outings, and the Red Sox bullpen where he went 1-0 with a 4.11 ERA (7 ER/15.1 IP) and 17 strikeouts in nine games. He last appeared for the club in Sunday's win at Detroit, earning a hold in 1.0 inning of work. For his major league career, Masterson, a 2013 American League All-Star, has gone 64-74 with a 4.31 ERA (575 ER/1201.0 IP) and 1,004 strikeouts in 258 games (184 starts) for the Red Sox (2008-09; 2015), Cleveland Indians (2009-14), and St. Louis Cardinals (2014).
Betts, 22, was placed on the seven-day concussion disabled list on July 29. He made one rehab appearance with Double-A Portland on Sunday, going 2-for-4 with a solo home run as the Sea Dogs' leadoff hitter and center fielder. The right-handed batter has hit .265 (103-for-388) with 52 runs scored and 48 RBI in 97 games for the Red Sox this season, leading Boston in doubles (25), stolen bases (13), and plate appearances per strikeout (8.35). In 149 career major league games over the last two seasons, the native of Tennessee has a .274 batting average (158-for-577), 37 doubles, seven triples, 15 homers, 66 RBI, and 20 stolen bases. He was selected by the Red Sox in the fifth round of the 2011 June Draft.
Over his two major league stints with Boston this season, Marrero, 24, appeared in six games, going 1-for-7 with a run scored while making three appearances (one start) at second base, one at shortstop, and one as a pinch runner. He made his big league debut for the Red Sox on June 28 at Tampa Bay and last appeared for the club on Saturday night in Detroit. For Pawtucket this year, Marrero was hitting .237 (65-for-274) with 11 doubles, one triple, five home runs, and 22 RBI in 78 games. Defensively, he has appeared in 70 minor league games this year at shortstop, seven at second base, and one at third base.
Cook, 28, made two relief appearances for Pawtucket after being optioned on August 4, recording three strikeouts in 2.1 innings and retiring all seven batters faced. The right-hander did not appear in a game in his first stint with Boston after being acquired from the Oakland Athletics on July 31 in exchange for a player to be named later or cash considerations. Cook has spent the majority of this season with Oakland's Triple-A Nashville affiliate, going 4-1 with eight saves, a 4.05 ERA (15 ER/33.1 IP), and 26 strikeouts in 30 relief outings. A member of the 2012 American League All-Star Team, Cook is 13-12 with 17 saves, a 2.93 ERA (66 ER/202.2 IP), and 207 strikeouts in 212 career major league outings, all in relief, between the Arizona Diamondbacks (2011) and A's (2012-15). He was originally selected by Arizona in the 27th round of the 2008 June Draft.
Uehara, 40, suffered the injury while making the final play of Friday night's 7-2 victory in Detroit, a line drive off the bat of Ian Kinsler that struck him on the right wrist. In 43 games this season, he is 2-4 with a 2.23 ERA (10 ER/40.1 IP) and 47 strikeouts compared to just 28 hits and nine walks allowed. He ranks among American League relievers in saves (T-5th) and WHIP (10th, 0.92). Uehara has successfully converted each of his last 15 save chances dating back to June 2, the third-longest active streak in the AL. Over 337 career major league outings (12 starts) in seven major league seasons between the Baltimore Orioles (2009), Texas Rangers (2011-12), and Red Sox (2013-15), the native of Japan is 17-19 with 86 saves, a 2.42 ERA (105 ER/390.2 IP), and 459 strikeouts.