Hefner took himself out of his third rehab start with the Port St. Lucie Mets last Wednesday after feeling discomfort in the arm.
"I'm not sure what my course of action is," Hefner wrote in a Facebook post. "Should have a clearer picture in the coming days. I'm disappointed, but not shaken. I have a lot of things to be thankful for, and I'll rest in that."
In 50 career appearances (36 starts) over two seasons, Hefner is 8-15 with a 4.65 ERA.
Manny Randhawa is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.