The right knee inflammation is more of a concern at this point for Rivera because it is his pushoff leg.
"I always saw [Hideki] Matsui with the knee, and I never experienced something until now," Rivera said. "I understand now how it is. Definitely, I have to take care of it."
But the side injury is also nagging -- Rivera compared it to right-hander Sergio Mitre's similar oblique strain, which landed Mitre on the 15-day disabled list, and Rivera said it felt like "more than an ache."
Rivera has a 1.08 ERA in 33 appearances, including 19 saves in 21 opportunities. While he has quietly been receiving treatment, Rivera entered Tuesday having pitched in four of New York's last five games, including a 1-2-3 save on Monday at Oakland.
"It doesn't get better like that," Rivera said. "I've been pitching and pitching and pitching. It's hard to rest it and get healed by itself."
Rivera was a player vote selection to the pitching staff, his 11th time on an All-Star roster, so he will be replaced by the next-highest-rating player in that balloting.
"It's hard -- it's a decision that I have to make for myself and the sake of the team," Rivera said. "It's a great honor, but I have to be wise."
It is possible that Rivera's problems could have been aggravated by a pair of two-inning outings on June 23 at Arizona and June 27 at Los Angeles.
"Definitely, there's no more two innings from here to the All-Star break," Rivera said. "I can't do it. I can't pinpoint to the two outings that I had, but definitely it's not the same when you're pitching one inning or two innings."
Rivera said that he was not sure if he would be available to pitch in the Yankees' remaining games before the All-Star break, saying that the decision would be up to manager Joe Girardi.
"I always want to pitch," Rivera said. "Even with one arm, I'd always want to pitch. It's something that we have to take care of. We haven't taken care of it enough. We did for a little bit, but it's there."