Randy Moss on NFL Network (edit- different Randy Moss, a reporter) reported that Marrone was given confidence by the doctors prior to the procedure that Manuel would likely be ready to practice in the week leading up to opening day. He went on to say that as Marrone was driving in Saturday morning he was debating whether he'd seen enough to announce the starting quarterback (did not specify who that was, but it may be obvious).
This aired right after the Bills broadcast ended on an NFL Network Now update