Evan Drellich of MassLive.com reports that Aceves and the Red Sox disagreed last week about whether Aceves was healthy, with the pitcher claiming that he had an oblique injury that might keep him out as long as the rest of the season, and the Red Sox denying Aceves had any serious injury trouble.
Aceves has a 4.86 ERA in 37 innings with Boston this season, with 5.8 K/9 and 5.4 BB/9. He has made six starts, with the last of those coming June 18. He is earning $2.65MM in his second year of arbitration eligibility.
Aceves was not eligible for free agency until after the 2014 season, so any team that claimed him on waivers would have had the option of taking him to arbitration in the fall. Also, any team that claimed Aceves would have been able to option him to the minors while keeping him on their 40-man roster. Despite Aceves' ability to start and the roster flexibility he would have offered, no one claimed him. As Britton notes, that suggests that other teams don't think particularly highly of Aceves, who has struggled with poor peripherals and reduced velocity this season and who has clashed with his managers in the past.