President of Baseball Operations Dave Dombrowski made the announcements.
Ysla, 23, will be assigned to High-A Salem and will work in relief. This season, he has pitched for San Francisco's High-A affiliate in San Jose, going 3-6 with a 6.21 ERA (55 ER) in 33 games, including nine starts. In his last 18 relief appearances dating back to the start of June, the native of Venezuela has a 3.65 ERA (15 ER/37.0 IP) with 47 strikeouts. Overall on the year, the left-hander has struck out 95 batters in 79.2 innings for a ratio of 10.7 strikeouts per nine innings. Originally signed by the Giants as an international free agent in September 2012, Ysla earned post-season All-Star honors in each of his first two minor league seasons with the Rookie-level Arizona League Giants in 2013 and Single-A Augusta in 2014. Over those first two minor league seasons, he combined to go 10-7 with a 2.51 ERA (48 ER/172.0 ER) in 36 outings, all but one as a starter. He entered the season as the No. 21 prospect in the Giants' system, that organization's third-highest rated pitcher.
Cook, 28, returns for his third stint with Boston, all since being acquired from the Oakland Athletics on July 31 in exchange for a player to be named later or cash considerations. He has allowed nine earned runs on 10 hits in 2.0 innings of work over three relief outings for the Red Sox. The right-hander has not allowed a run in seven bullpen outings for Pawtucket, recording 10 strikeouts while permitting just four base runners (two singles, two walks) in 9.1 innings. A California native, he 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, he is 13-12 with 17 saves, a 3.30 ERA (75 ER/204.2 IP), and 210 strikeouts in 215 career major league outings, all in relief, between the Diamondbacks (2011), A's (2012-15), and Red Sox (2015). He was originally selected by Arizona in the 27th round of the 2008 June Draft.
Craig, 31, appeared in 24 games for the Red Sox this season before being optioned to Pawtucket on May 9. In 93 games for the PawSox this season between first base (46 games), right field (21 games), and left field (2 games), he hit .274 (94-for-343) with 14 doubles, four homers, 30 RBI, and 49 walks. His .367 on-base percentage entering play on Monday ranked third among International League leaders. With Boston this season, he hit .135 (7-for-52) with one home run, two RBI, and five runs scored. Defensively, Craig has made 10 starts for the Red Sox in 2015, with five in right field, four in left field, and one at first base. Acquired in a trade with the St. Louis Cardinals on July 31, 2014, the right-handed batter has hit .278 (501-for-1,804) with 106 doubles, three triples, 59 home runs, 295 RBI, and 238 runs scored in 522 games between St. Louis (2010-14) and Boston (2014-15). Craig's .354 (136-for-384) batting average with runners in scoring position is second-highest among major league players over the last four seasons (2012-15), trailing only Miguel Cabrera (.364).
Leon, 26, played in 33 games for the Red Sox this season before being designated for assignment on July 20. With Boston, he went 16-for-89 (.180) with seven runs, a double, and three RBI, making 28 starts behind the plate and throwing out 56 percent of attempted base stealers (9 of 16). He also appeared once at third base. After clearing waivers, he was outrighted to Pawtucket, where he batted .263 (26-for-99) with four doubles, one home run, 13 RBI, and 10 walks in 26 games. Acquired from Washington on March 30, he has played parts of four major league seasons between the Nationals (2012-14) and Red Sox (2015), hitting .185 (34-for-184) with 16 runs, four doubles, a homer, eight RBI, and 17 walks in 67 games. The native of Venezuela has caught 15 of 31 attempted base stealers (48%) in his big league career.
This is the third career major league stint for the 25-year-old Ramirez. In three games with Boston, all in July, he is 0-1 with a 5.40 ERA (3 ER/5.0 IP) and two strikeouts. With the PawSox this season, he has gone 4-1 with three saves, a 2.32 ERA (11 ER/42.2 IP) and 38 strikeouts in 30 games (one start), including a 1.82 ERA (8 ER/39.2 IP) out of the bullpen. He has not allowed a run in 15.1 innings spanning 12 games at Pawtucket's McCoy Stadium. The California native has held right-handed Triple-A batters to a .188 average (15-for-80). Signed by the Red Sox as a fourth-round selection in the 2011 June Draft, Ramirez is 11-11 with a 2.87 ERA (86 ER/270.0 IP) and 251 strikeouts in 124 career minor league outings (17 starts).
De Aza, 31, played in 60 games for the Red Sox after being acquired from the Baltimore Orioles on June 3. With Boston, he hit .292 (47-for-161) with 18 extra-base hits (nine doubles, five triples, four home runs), 25 RBI, 12 walks, 23 runs scored, and three stolen bases. In 90 major league games this season between both teams, the left-handed batter posted a .261 batting average (69-for-264) with 13 doubles, six triples, seven homers, and 32 RBI while appearing in left field (52 games, 38 starts), right field (37 games, 29 starts), and center field (1 game, 1 start). A veteran of eight major league seasons with the Marlins (2007, 2009), White Sox (2010-14), Orioles (2014-15), and Red Sox (2015), De Aza has hit .267 (593-for-2,218) with 116 doubles, 29 triples, 45 home runs, 221 RBI, and 84 stolen bases in 656 career major league games.
With these moves, Boston's active major league roster is now at 28, and the team's 40-man roster is at 39.