Rivera said he first began to feel sore on Aug. 15 in Seattle. The right-hander has pitched in five games since then, including Monday and Tuesday, when he recorded his 37th and 38th saves against Baltimore.
"I've had groin issues, but this is new. It's the left leg, which is good because you don't push off that leg," Rivera said. "It just happened while I was pitching. It's nothing to worry about; we'll go day by day. I'll play as soon as possible."
Yankees manager Joe Girardi said Rivera was not going to pitch Wednesday regardless of the score.
"He's had it a little bit earlier and it went away and it was a little stiff after the game [Tuesday] night, so we just thought we're not going to use him tonight and we'll try to get rid of it," Girardi said, adding that he might keep Rivera out of the four-game series in Toronto that begins Thursday.
"It's a concern. You don't want to hurt him," Girardi said. "So we thought we'd give him a couple days and see if we can nip it in the rear end. I don't even know if I would call it a strain, it was just a little tight last night. You kind of give him a few days and see if he can get rid of it."
The 39-year-old Rivera leads the majors in saves and has converted a personal-best 34 in a row.
Phil Hughes, who earned the save in Wednesday night's 10-2 win over the Orioles, will serve as closer in Rivera's place, Girardi said.