Denver Broncos quarterback Peyton Manning broke the all-time NFL record for passing on Sunday against the Kansas City Chiefs, surpassing the league-leading mark set by Brett Favre. In the same game, Manning, now 39, gave an overall performance that seemingly offered a very telling glimpse into his, and the Broncos', immediate future at quarterback. After breaking the record, Manning proceeded to toss four interceptions, amassing a 0.0 quarterback rating and just 35 yards through the air before being replaced by long-time back up Brock Osweiler.

And while there's simply no doubting that Manning's best days are certainly behind him, and this season is likely to be his last in an NFL uniform, we may now have a legitimate reason for why Manning performed so poorly.

According to a report from ESPN's Adam Schefter, Manning suffered a torn plantar fascia in his left foot. The future Hall of Famer apparently was battling plantar fasciitis going into the game, only to have the issue worsen after kickoff. It was first reported last week by Vic Lombardi of CBS Denver that Manning was dealing with plantar fasciitis, but it seems his condition became more severe Sunday.

Of course, an injury like this - or, really, any injury at this point - likely offers little consolation for Broncos fans. Manning, at his advanced age, has become very fragile over the past few seasons. While he's missed little actual time on the field, he has struggled in games, his throws lacking the velocity necessary to beat outside coverage.

During Denver's upset loss to the Indianapolis Colts in the first-round of last year's playoffs, Manning looked woefully overmatched, tossing 26 completions on 46 attempts for just 211 yards and one touchdown.

While Osweiler managed 146 yards and a touchdown in place of Manning, he also tossed an interception and failed to show signs of being a potential franchise quarterback. Then again, being tossed into such an unfavorable situation as he was, with the game already out of reach, he'd have been hard pressed to make an overly positive impression.

In the end, it will be interesting to see what direction Denver's season takes after Manning's injury. Can Osweiler take command of the top spot? Or will he prove to be not ready for the spotlight?

And is this it for Manning, one of the game's all-time greats?