Shortly before Monday's Grapefruit League game against the Orioles, it was announced that Thornburg had an upper right trapezius spasm.
When the Red Sox acquired Thornburg from the Brewers in December, the plan was that he would be the team's main setup man in the eighth inning.
But after two rough Grapefruit League outings and subsequent arm fatigue which caused his near month-long shutdown from pitching in a game, manager John Farrell had said in recent days that Thornburg would start the season in lower-leverage outings.
Joe Kelly will probably begin the season pitching the eighth inning, but Heath Hembree and Matt Barnes could also get opportunities to set up Craig Kimbrel.