If he doesn't win Super Bowl, Peyton Manning will be remembered as football's nearly man. However, not claiming as many Lombardi Trophies as he should have won't be Manning's abiding legacy.
Redefining the way defense is played in the modern game and expanding the requirements for today's quarterbacks will be Manning's true legacy.
Note that it is defense where the impact of arguably the greatest quarterback of his generation is felt most. For no other quarterback has done more to influence modern defensive football than Manning.
Defense is a reactionary process, no matter how often coordinators talk about wanting to be aggressive. Defensive trends often come in response to offensive evolution.
The genesis of today's pass-happy, spread system league can be traced back to 1999. That year the St. Louis Rams stunned the league thanks to Mike Martz and his multi-receiver, spread-based attack.
Teams began to see the possibilities of spreading formations to the breaking point with speedy receivers. At the same time the Rams were lighting up scoreboards, a second-year quarterback was making waves in Indianapolis.
Manning went 3-13 with the Colts as a rookie in 1998. In '99, he engineered a swift and remarkable turnaround to 13-3.
That year the Colts ranked fourth in total offense and passing yards. Manning threw for over 4,000 yards and 26 touchdowns. Few expected him to so dominant, so fast.
What was more impressive was how he did it. The Rams were dissecting opponents with lightning-quick precision.
There scheme was specific and meticulously designed. Manning was more fluid. He was spotting gaps in coverage and anticipating matchup advantages pre-snap.
While the Rams didn't really care what opponents did, Manning tailored his offense to exploit exactly what defenses were giving him.
After the '99 season, the passing game expanded at an alarming rate. Yet while the scheme-specific Rams could never sustain the success of that year, Manning went from strength to strength.
In the process, he forced defenses into an entirely new way of thinking. Because there was no specific system to attack when facing Manning, defenses had to become just as fluid as him.
That meant mixing fronts, shifting personnel and disguising coverages. Everything from rotating safeties and faking the blitz were responses to the many adjustments Manning would make pre-snap.
The battle between offense and defense used to pit concept versus concept—for instance, how well a gap-control front repels a zone-rushing attack. That could form the pattern for a whole game.
Manning changed the concept struggle on almost every individual play, and forced defenses to keep up. Coordinators were no longer able to rely on favored alignments and game plans.
"Mix it up or die" became the mantra for defensive football. In many ways, Manning obliterated traditional defensive looks.
He helped make versions of the nickel the de facto base defense for most of the league. Manning started that trend because defenses were trying to keep up with him.
Combating a quarterback who could immediately identify weaknesses in the base look, or would change plays twice at the line, required new approaches.
Disguise became everything in modern defense. Hiding pressure until the last second and showing false coverage looks were key.
The New England Patriots had great success against Manning because they mastered these dark arts. Bill Belichick's hybrid fronts and late rotations in coverage were designed to give Manning the same headaches he had caused for many defensive coordinators.
When they saw how the Patriots coped with the league's most dominant passer, the rest of the league followed suit. But the trend Manning started didn't just alter defenses, it changed things for every quarterback that entered the league after '98.
Manning made being a field general fashionable again. For a quarterback position that had become over-coached, Manning returned some level of autonomy to the men under center.
Rather than simply resembling string puppets for clever offensive coordinators, quarterbacks post-Manning became their own coordinators.
When other teams saw what Manning did with a free rein to manipulate his own schemes, they wanted their quarterbacks to do the same.
Simply being a game-manager, or even a possessing a strong arm, is not enough anymore. Quarterbacks have to be able to adjust on the fly.
And not with the gung-ho instincts of so-called "street ball" either.
Today's signal-callers must be cerebral in their approach. They must react with the intelligence of master strategists.
Current offensive and defensive designs demand quarterbacks capable of plotting counter-move after counter-move. The NFL's top quarterbacks are all defined by this quality.
That explains the prevalence of the fast-paced, no-huddle attacks and the chameleon-like defenses created to stop them. Manning was the catalyst for that dynamic.
If he fails to lift another Super Bowl, critics will point to the lack of rings. However, this is an overstated view and one that should not define Manning.
The likes of Fran Tarkenton and Dan Marino were true greats of the game. Yet neither has or needs the prizes to endorse that label. In the same way, the real definition of Manning's greatness lies elsewhere.
Manning changed the way the defense is played in the modern era. At the same time, he also expanded the minimum requirements for greatness at his own position.
That will be the true legacy of No.18.