Armstrong is dealing with tightness in his right lat and will sit out a few days, Jordan Bastian of MLB.com reports.
The issue doesn't appear to be too serious, so he should head back to action soon. Armstrong pitched well last season with the major league club last season, so he's got a good chance to make the Indians roster again in 2016. However, he'll have to stay healthy in order to prove himself to the Cleveland brass and ensure himself that spot.