Manning tossed four interceptions while leading the Broncos to just three first downs in two and a half quarters of action. Never comfortable in the pocket against an improved Kansas City defense, he completed just five of 20 pass attempts for 35 yards and a 0.0 passer rating.
Although a clearly diminished Manning has had his share of ugly performances dating back to last December, Sunday's performance was easily the most disturbing.
The foot injury that limited Manning in practice this week might have been a factor on several flutter balls, but the injury doesn't explain the poor reads that led to a couple of interceptions.
To his credit, Manning didn't blame his performance on any injury.
"I thought I felt good enough to play. Maybe that was wrong," he said, per NFL Media's James Palmer. "I thought I was good enough to help the team and ended up hurting the team"
Denver's convincing Week 8 victory over Green Bay seemed to be a turning point for Manning and the Broncos offense. Instead, Manning has regressed over the past two weeks, leaving coach Gary Kubiak in the unenviable position of deciding whether an NFL legend is the best option to lead his first-place outfit going forward.