Sunday's performance against the Kansas City Chiefs was painful to watch for any Denver Broncos' fan, but apparently nobody was in more pain than Peyton Manning, who struggled mightily.

We knew going into the game there were physical issues with Peyton Manning - a foot injury, sore ribs, shoulder. But there was never any indication that there was anything seriously wrong that would keep him from playing or from performing at a high level.

We now know that the foot injury he's dealing with is significant. According to the Denver Post, Manning is suffering from plantar faciitis, a very painful foot injury. Manning apparently first began experiencing pain a couple of weeks ago, and then aggravated the injury a week ago against the Colts.

The injury includes inflammation and a partial tear of tissue and the prognosis for healing depends on the extent of the injury. It most likely would require more than just a week to fully heal. The feeling of pain has been described by some as a spike being driven up through the heel when walking. Ouch!

The injury is in the right foot - an essential element for a quarterback trying to throw the football - and it clearly was affecting Manning's ability to throw. He thought he was well enough to play, but, obviously, that wasn't the case.

What does the future hold? Only time will tell. It seems the best course of action would be to let Peyton rest this week and try to get healed up. Let Brock Osweiler take the start this week against the Bears, and hopefully, Manning is good to go the following week against New England.

UPDATE: Denver Broncos Head Coach Gary Kubiak announced Monday Peyton Manning will not play Sunday against Chicago and there is no timetable for his return. Brock Osweiler will make his first career start in a regular season game.