I don't think Manning will have his way at all with the Bears. Urlacher, as you know, a LB, pretty much covered Reggie Bush the whole game, except for the illegal pick play. Don't forget about Briggs and the rest of the Bears D. They keep getting disrespected and I LIKE THAT. The Bears remember that the Colts kicked their butts a few years ago, it's already all over the radio in Chicago. If the Bears run all day long, add a short passing game, and eat up the clock, Manning isn't going to get his hands on the ball very much (ala CW/ND vs. USC 2005).
Second, I don't think Indy's D is even close to as good as the Bears D. The Bears will force turnovers by Manning with disguised coverages. Brees had 4,000+ yards passing and it did him no good, the same will happen to Manning. Manning had better get ready for a good old fashioned Ass-whooping Chicago Bears style. They are going to physically punish him and get him mentally out of the game.
Yes, Grossman could screw it up, BUT, he is a streak player. If he gets hot and the running game is working, the Colts can go home early. Remember, the Bears didn't let Grossman blow the game, their running game and D controlled all. A strong dose of Benson and Jones will tire out the Colts by the middle of 3rd quarter.
Bears 31 Colts 24