Mariano Rivera has experienced inflammation in his right elbow and will have a precautionary M.R.I. on the elbow soon, perhaps on Thursday. After working two innings for a save on Aug. 20, Rivera did not pitch for six days, until another two-inning outing on Sunday. There was only one save opportunity in that stretch, and Rivera said yesterday that he felt good and that he had enjoyed the rest. Asked if the innings add up this time of year, Rivera smiled and said, ?They do, they do.?