The retirement of Baltimore Ravens linebacker Ray Lewis shouldn't merely have ignited debates about where his legacy ranks in NFL history. It also should've sparked a few more questions about how Denver Broncos quarterback Peyton Manning -- the man Lewis will be hunting in Saturday's AFC divisional playoff game -- will be handling the end of his own career. Lewis seemingly found peace with his decision at some point this season. Manning will have a tougher time determining how much longer he wants to stick around.

This entire season for Manning has been about one thing -- proving to everybody who watches pro football that the man can still play at a high level. That was a major concern when he was coming off four neck surgeries and making his former team, the Indianapolis Colts, squeamish enough to cut him. Not so much anymore. Manning probably will walk away with league MVP honors and, if he's lucky enough, a second Super Bowl ring to boot.

It would be an amazing story if all those things really happened for a star who was once so down and out. It would be even more incredible if Manning were able to sustain that brilliance for a few more years in Denver. The reality is that neither is guaranteed. This might be as great as we ever see Manning play for the remainder of his career, and that's why it's worth wondering how this is all going to end for him.

Peyton Manning has proved plenty of doubters wrong this season. Mark Reis/Getty Images

This isn't an issue of talent, by the way. It's one of motivation. Manning, like most great athletes past their prime, found the most valuable inspiration a player could use and rode it for all it was worth: doubt. The minute the Colts, the media and the general public started questioning whether we'd ever see him play again, something had to have ignited in the man. The last thing he ever was going to do was go out with a whimper.

That desire drove Manning through his rehabilitations and his top-secret workouts and all the hype that surrounded his arrival in Denver. It also meant plenty when critics knocked his declining arm strength earlier this season, when it seemed that a 40-yard throw was as taxing as to him as launching an anvil across the field. But what Manning will discover after this season is that such motivation lasts for only so long. At some point, his time in Denver will have to be about more than proving everybody else wrong.

Brett Favre learned that lesson the minute he left Green Bay. He bounced first to the New York Jets and then to the Minnesota Vikings, all in the hopes of exacting some level of revenge on the Packers for trading him to clear room for Aaron Rodgers. That combination of vindictiveness and focus drove Favre to arguably the best season of his career in 2009, when he led the Vikings to the NFC Championship Game and two victories over Green Bay. But, as we soon discovered, anger works for only so long when it comes to finding reasons to play this game.

Favre eventually bottomed out with the Vikings the next season, and it's worth arguing that lack of conviction was one of his major issues. He had become a mercenary working for money and pride, an icon disconnected from the place where he had enjoyed his favorite years. We'd never see Manning turn his career into the circus Favre's eventually became, but there is one big red flag to consider. Like Favre, Manning has reached the point in his life when there aren't any grand prizes he hasn't already claimed.

Brett Favre had trouble, literally, walking away from football. AP Photo/Michael Dwyer

Manning has four MVP awards. He has played in two Super Bowls. A first-round Hall of Fame ballot is a certainty, and only New England's Tom Brady can challenge him as the top quarterback of his generation. About the only thing Manning hasn't done is break any of Favre's NFL career passing records. At his current pace, he'd need about three more good years to become the league's career leader in completions, yards and touchdowns.

It's hard to know whether those records are enough to drive a man who turns 37 in March and missed the entire 2011 season. We can't predict whether Manning's passion for the game will continue to burn as his time in Denver passes. He had a deep bond with the city of Indianapolis, the same connection Lewis had in Baltimore and Favre shared in Green Bay. It's different when stars move on to other towns late in their careers. Those same little things that move them with their first teams --the familiarity, the comfort, the trust -- matter more than you imagine once lost to a player who sees 40 on the horizon.

Don't for a minute think that Manning won't be affected by the absence of some of these things the longer he stays in Denver. He spent most of the past year trying to come back from neck surgery, most of last winter determining his future in Indianapolis and most of this past offseason preparing for this season in Denver. There's been precious little time for reflection. If anything, Manning has preferred it that way.

At some point, that will all change. He'll have to ponder the same things Lewis has been considering in recent weeks. He might face the same challenges Favre faced in his second season in Minnesota, when the Vikings underachieved and that feel-good story didn't feel so good anymore. For all we know, we might never see Manning this healthy again in the remainder of his career.

Remember, most star quarterbacks don't last very long once they leave their first franchises. Joe Montana strung together two years in Kansas City after his time in San Francisco. Donovan McNabb spent one season each in Washington and Minnesota -- both abysmal campaigns -- after his days in Philadelphia. Favre was out of football within three years of Green Bay trading him.

Manning might be a unique breed, but it's a reach to say he'll be any different from most. He has found the right reasons to keep playing and has reminded us of what makes him great.

Now comes the really hard part. He has to figure out what to do once he has silenced all his critics.