We don't know much about quarterbacks in Chicago; how could we if the best one we ever had played so long ago he wore a leather helmet?

We don't know much about receivers either; Brandon Marshall is like a Martian in the context of Chicago's football history.

But linebackers we know. Great linebackers we've watched in abundance. George, Butkus, Singletary, Urlacher. It's a linebacker's Mount Rushmore. Linebackers are to the Bears what centers are to the Lakers: Mikan, Wilt, Kareem, Shaq.

Brian Urlacher was an eight-time Pro Bowler and Defensive Player of the Year in 2005. Scott Boehm/Getty Images

If you ask me, Brian Urlacher should be playing at least one more year, and for the Bears at that. It doesn't matter that he doesn't run as fast or as well as he used to; who in the NFL does run as fast or as prettily at 34 as he did at 22 or 30?

Urlacher, even in decline, forced fumbles, returned interceptions, inspired teammates, and we're talking about last season. When I wrote last summer that Urlacher, coming off knee surgery, should wade carefully into the 2012 season, bag September and try to play the final 12 games or so, Julius Peppers told me I was nuts, because it was Urlacher who was the player who was going to get Bears off to a good start (which he did). It was Urlacher who was going to have the defense playing at a championship level (which he did for the first half of the season). It was Urlacher who was the heart and soul of not just the defense but the entire team (which he was).

With Urlacher, diminished as his detractors said he was, the Bears were contenders, and nothing more than a threat without him. The Bears' takeaways, opponents' points, opponents' third-down efficiency and opponents' total quarterback ratings were all worse -- much worse -- in the final four games of the season. His impact, right up to the end and even on the sideline, was undeniable. He played with Singletary's intelligence and Butkus' menace.

The natural tendency is to say Urlacher is going to be irreplaceable, except that this is where the Bears, almost magically, come up with a capable successor. Just as the Bears found George Connor to replace Bronko Nagurski, and Bulldog Turner to replace Connor, and Bill George to not only replace Turner but essentially invent the position of middle linebacker as we've come to know it, Dick Butkus replaced George. And after Butkus retired in 1973 it took awhile, but Mike Singletary, almost unthinkably, replaced Butkus ... in impact if not in destruction.

During the years I covered the NFL for The Washington Post, and even since joining ESPN, the retirement of a great player was an occasion for me to call Steve Sabol of NFL Films, for my money the Bert Sugar of pro football and one of the great historians of American sports. Nobody could put great players in perspective any better than Sabol.