Newton suffered a hairline fracture in his rib during Friday night's 30-7 preseason loss at New England. He will not play in Thursday night's exhibition finale at Pittsburgh.
But judging by the way Newton moved around on Tuesday, Rivera was encouraged.
"I saw him throw a couple of balls, I saw him catch a couple of balls, I saw him bend over and pick up [a couple of balls], so I imagine it is lessening," Rivera said of Newton's pain.
"This is exactly what the doctor told us. It's just a matter of time. Get the stiffness out, the soreness out, and he'll be all right."
While Newton has been held out of practice this week, he has been on the field going through the mental reps with players and coaches.
Asked if he had no doubt Newton would be ready for the Buccaneers, Rivera said, "I'm pretty confident. The true evaluation will come in about a week, so we'll probably be looking at this Saturday for where he is and how he is."