The club did not confirm the signings.
Acosta, who ranks No. 6 on MLB.com's Top 30 International Prospects list, and Espinoza, who ranks No. 10, are among the best pitchers on the international market this year, and they will be a welcome addition to the club's Minor League system.
Acosta's fastball hovers in the 88-92 mph range, while his changeup might be his best pitch. The 6-foot-3, 170-pound Acosta also has an effective curveball that has a good rotation and bite; he uses it in the strike zone both early in counts and as a strikeout pitch.
The 16-year-old has been praised for his command and his overall feel for pitching. Acosta is still fine-tuning his delivery. He is based in La Vega, Dominican Republic, and is trained by Alberto Faña.
Espinoza has three projectable pitches with command, and he's a plus-athlete who has a chance to be a top-of-the-rotation starter. The right-hander's fastball hovers in the 91-93 mph range, and his curveball comes in around 71-73 mph. Espinoza's changeup also has some sinking action.
In accordance to the Collective Bargaining Agreement, each team is allotted a $700,000 base and a bonus pool based on the team's 2013 record for the international signing period, which started Wednesday. Boston's bonus pool total for this year's signing period is $1,881,700, and the team will be penalized for exceeding its spending limits.
Teams that exceed the pools by 0 to 5 percent have to pay 100 percent tax, and teams that exceed the pools by 5 to 10 percent are not allowed to sign a player for more than $500,000 during the next signing period and have to pay a 100 percent tax on the pool overage. Teams that exceed the pools by 10 to 15 percent are not allowed to sign a player for more than $300,000 during the next signing period and have to pay a 100 percent tax on the pool overage.
In the most severe penalty, teams that exceed the pool by 15 percent or more are not allowed to sign a player for more than $300,000 during the next two signing periods, in addition to paying a 100 percent tax on the pool overage.