Newton underwent surgery on his ankle last week and is expected to be out for up to four months, which will rule him out of his side's organised team activities in May.
"They went through what they do normally," Rivera told the Charlotte Observer. "Every year it's been the same thing for the last couple years. This time when he started, after the rest and everything, it wasn't getting as well as fast as it had last time.
"They went through the [process], looked at it and decided this was the best course of action for him."
Newton threw for 3,379 yards along with 24 touchdowns to 13 interceptions in the 2013 season.