He said he first felt discomfort in the heel before pitching one inning Saturday at PNC Park.
"It kind of felt like a knife was going in the back of my heel," Qualls said. "I threw yesterday with it, but I really couldn't move too well. It was still really bugging me, trying to walk today. So they just said they were going to try to stay away from me today. Hopefully it's nothing too big. If I had to, I could have gone out there and thrown, but I think being this early in the season, they just want to be cautious."
Qualls said he will see the team doctor Monday.
"I really don't know what it is," he said.