It's likely that Manning was just being given a day of rest. He's expected to return to practice on Thursday and is in no danger of missing Sunday's game against the Washington Redskins. Wide receivers Wes Welker and Eric Decker were also given the day off and should all be back on Thursday. At worst, Manning might end up on Friday's injury report with the ankle issue.
Aside from multiple neck surgeries that cost him the 2011 season, Manning hasn't missed a single game in his career. Through seven games, he's on a record-setting pace with the 6-1 Broncos, throwing for 2,565 yards, 25 touchdowns and just three interceptions.
In the unlikely event that Manning is unable to play Sunday, Brock Osweiler will get the start. He hasn't played much outside of garbage time in his two-year career.