ENGLEWOOD, Colo. -- The big "if" arrived at the Denver Broncos' doorstep Monday morning.

It revolved around the revelation, or confirmation, of the rumblings that have made their way through the Broncos’ locker room in recent weeks: that quarterback Peyton Manning has a partially torn plantar fascia in his left foot. It sets the wheels in motion for the Broncos' short- and long-term quarterback situation. For the short term, it's who will be behind center for the Broncos on Sunday in Chicago, with the long-term question focused on who that will be for the rest of the season and beyond. It’s clear Manning wasn’t right Sunday; it was his worst day, punctuated by his benching in favor of Brock Osweiler.

Manning, who set the career passing yardage record Sunday, was 5-of-20 for 35 yards with four interceptions. And while he looked OK as the team warmed up before Sunday’s loss to the Kansas City Chiefs, there was a moment just before they went back to the locker room before kickoff when whatever the Broncos had done to get Manning ready to play was negated.

Manning looked like he was in pain and frustrated. He didn’t plant his left foot after that -- the way a right-handed quarterback needs to plant to play professional football. And as an afternoon framed in history simply eroded into one of his worst career days, Manning’s foot was obviously bothering him, to the point where he was doing what he has said he doesn’t want to do.

Manning has always said he plays because he enjoys it, all of it: practices, meetings and games. And he plays to help his team win. And when his 39-year-old left foot betrayed him, along with a sore right shoulder, sore ribs, a neck that has been surgically repaired four times, and a knee that requires a brace for support after he had an infected bursa sac removed from it earlier in his career, Manning was neither enjoying himself, nor was he helping his team win.

After Sunday’s game, Manning was asked about the fact that he looked more comfortable in warm-ups than when the game started.

“Oh, I can’t, I don’t know,’’ Manning said with a sigh. “I can’t say that; I guess that’s an observation. I just didn’t -- I thought I felt good enough to play, that’s what I thought, maybe that was the wrong -- maybe that was a false feeling by me or whatever that was. ... I was honest with them with how I felt, thought I was good enough to play. Maybe looking back that was the wrong, um ... I had the wrong indication by me and by going out there trying to help the team, I ended up hurting the team. I’m disappointed about that.’’

And that is where the "if" comes in, and it's a big one. Because Broncos coach Gary Kubiak said quickly and with emphasis Sunday that "if he’s healthy and ready to go, Peyton is our quarterback." Well, Manning isn’t healthy and he’s clearly not ready to go. What does it all mean? Will Manning play in a couple of weeks? Several weeks? At any point for the remainder of the season?

These are questions that will be answered by Kubiak, John Elway and the rest of the Broncos organization at some point. They are tough questions with difficult answers.

Manning is a Mount Rushmore player in the NFL. He returned from spinal fusion surgery to throw for 5,477 yards and 55 touchdowns in 2013, simply one of the best seasons ever constructed. His résumé is one of a kind. And now the question becomes: Is Manning’s left foot injured severely enough that Sunday's performance was the last look at one of the best ever?