The good news is that it sounds like a short-term injury for Atchison, described as "tightness" in his forearm. It was enough to diminish Atchison's effectiveness, however, and the Sox opted to go with one of their minor league options. In this case: Junichi Tazawa.
While Tazawa hasn't pitched in the majors since the end of April, it's not for a lack of results. Pushed off the major league club by one of the many roster crunches the team has faced, Tazawa did not earn his departure, throwing six strong innings in five games. Since then, he's been doing just fine for himself in the minors, and currently holds a 2.55 ERA in 42 innings with an impressive 56 strikeouts and 17 walks.
Tazawa would struggle through a bit of a disappointing June--if only peripherally--but he's all-the-way back so far in July, striking out 11 batters in six innings that have seen a grand total of four baserunners. It'll be interesting to see how much of that he can carry back up to the majors.