Clippard had not thrown in five days because of tightness in his back. The righty setup man said he felt fine after an 8-minute bullpen session Monday.
Mattheus says he began experiencing discomfort following a bullpen session last Tuesday, and that it bothers him when he takes a deep breath, coughs or sneezes, or even rolls over in bed. The right-hander thinks he should only be out two weeks at most.
"It's kind of painful," he said.
The MRI set for Monday will determine whether the problem is a strained muscle or cartilage inflammation. Mattheus is competing for one of two open spots in Washington's bullpen.