Apparently, Rivera has not been available since Friday night, when he closed out the 6-4 win over the Chicago White Sox but felt the stiffness.
"It didn't affect me at all," Rivera said. "I finished pitching the game, but I felt it, you know? I can't even describe it. I didn't feel any pop or pull or anything like that. But it's like, when I sneeze, I feel it."
Rivera played catch Monday and will try to throw off a mound Tuesday. He is day to day.
"I'm not worried at all," he said. "Like I said, I threw today and it felt good but I didn't want to push it."
The injuries continued to pile up for the Yankees after catcher Jorge Posada left Monday's 4-1 win early with a mild right calf strain. Like Rivera, he is day to day.
"I think any time you start seeing guys get a little bit nicked up it's a concern," Yankees manager Joe Girardi said.
Information from ESPNNewYork.com's Wallace Matthews and The Associated Press contributed to this report.