Armstrong (back) was optioned to Triple-A Columbus on Saturday.
The training staff recently cleared Armstrong to return to live action after he missed more than two weeks with back tightness. His first appearance back was one to forget, as Armstrong gave up two runs on two hits and did not record an out against the Reds on Thursday. The 25-year-old averaged more than 14 strikeouts per nine at Triple-A last season and carried that success over to the major league level late in the year, so he should be among the first relievers called up in-season so long as he keeps the walks in check.