"We think it has something to do with the muscle in there," Saunders said on Monday. "Just to be safe, I'll have an MRI on Thursday. We're thinking, and hoping, that it's nothing major."
Saunders hurt the shoulder when he rammed it into the wall at Safeco Field trying to catch a ball hit by Rangers outfielder Nelson Cruz on Aug. 5.
Saunders has played five games since then, going 2-for-19, including an 0-for-4 collar on Saturday afternoon against the Yankees.
"It bothers me more when I swing than when I throw, and I think that's a positive sign," he said. "We're just treating it every day."
He will be available for pinch-running and defensive duty during the three-game series against the Red Sox.
"It's never fun to have these nagging things," Saunders said. "Hopefully it won't get any worse by playing."