"I've been told it's headed in the right direction," Rivera said of his treatment.
Rivera will undergo multiple weeks of follow-ups and scans after concluding chemotherapy and a proton treatment Monday, Keim adds.
The 58-year-old was diagnosed with squamous cell cancer in August. Doctors told him it was treatable and curable.
He said fatigue has been one of the most significant side effects of his treatment.
"How tired you get, at times you get nauseous," he said. "At times your equilibrium is messed around with, almost a sense of vertigo. And then the nausea. It hits you at any time, anywhere.
"But the fatigue, going out to practice it limited me, and that bothers me because I can't coach the way I coach."
Rivera hasn't missed any games due to his illness, but he has been forced to skip a few practice sessions. He previously said coaching through cancer was "a struggle."