LAKE FOREST, Ill. – Leave it to Mike Brown to deliver one of the bigger hits from the Chicago Bears recently.

While the slate gets wiped clean following Thursday's preseason finale at Cleveland, Brown made no effort to hide his concern after watching the defense fall flat against San Francisco in a 37-30 loss last week. Just an exhibition? Don't tell him that.

“That's not an excuse,” the former Pro Bowl safety said.

It didn't rank up there with Brown saying his team stinks after a 1-3 start in 2005, but the message was clear. If the defense is going to carry the Bears again, improvement is needed.

That was evident last Thursday.

After containing Kansas City and Seattle, the Bears' starters could not hold the 49ers in check despite an unsettled quarterback rotation and the absence of three starting linemen and three wide receivers. The worst offense in the league last season, San Francisco quickly jumped on the Bears (0-3 preseason) while piling up 27 points against the first-teamers in the first 2½ quarters.

The Niners' Frank Gore cut through a big hole on the first play from scrimmage for an 11-yard run and did it again on the second play, this time sidestepping Brian Urlacher on a 28-yarder.

Then, Urlacher got sealed off on a 16-yard run by Zak Keasey on the fourth play, putting the ball on the Chicago 16.

The 49ers started looking downfield on their second possession, and the Bears were unable to contain quarterback J.T. O'Sullivan, who threw for 126 yards and a touchdown in his limited time while clinching the starting job over Alex Smith. O'Sullivan was 7-for-8 with a perfect 158.3 passer rating, leading the Niners to field goals on their first two possessions before a two-play, 77-yard drive to the end zone on the third one.

That poor showing helps explain why Chicago ranks 31st in total defense (397.7 yards per game), pass defense (236.3 yards) and run defense (161.3). With one more tuneup before the opener at Indianapolis, the Bears certainly aren't panicking even though they are concerned.

“We're better than that,” coach Lovie Smith said. “It's kind of as simple as that. There's no need for panic or anything like that, but that's not acceptable and we'll play better defense in the future.”

In some ways, the Bears are trying to go back to the future. They're counting on their defense to lead the way, just as it did when they reached the playoffs in 2005 and Super Bowl in 2006.

They see last season as an anomaly, nothing more.

General manager Jerry Angelo showed his faith in the offseason by giving Urlacher, defensive tackle Tommie Harris and defensive end Alex Brown contract extensions while re-signing linebacker Lance Briggs even though the defense took a big step back last season.

Chicago allowed 354.7 yards per game and ranked 28th overall while managing just 33 takeaways last season after leading the NFL with 44 in 2006.

Injuries played a big role.

Mike Brown and starting defensive tackle Dusty Dvoracek ruptured their left Achilles' tendon in last season's opener at San Diego, and the injuries kept coming.

Cornerback Nathan Vasher played just four games because of a groin injury and cornerback Charles Tillman, Briggs, defensive end Mark Anderson, and departed safety Adam Archuleta and tackle Darwin Walker all missed at least one game.

Urlacher and Harris were hurt, too, although they played in all 16.

Urlacher had an arthritic back and needed minor neck surgery after the season while Harris, coming off a season-ending hamstring injury in 2006, was bothered by a sprained knee last year.

“I'm always concerned if we're not performing the way we're supposed to be, but I'm confident that we're capable of doing it, of being a dominant defense and we will be,” Briggs said.

The Bears got off to a bad start in the first preseason game, when Kansas City's Brodie Croyle led the Chiefs 81 yards for a touchdown on the opening drive. The starters forced a punt on the next possession before being lifted, and they shut down Seattle in the second game, although the Seahawks used third-string quarterback Charlie Frye the whole way.

Then, the Bears crumbled against San Francisco.

“We're not panicking,” Alex Brown said. “I think if we'd have played this game and we just dominated San Francisco, everybody would have said, 'It's just a preseason game. Let's do that against Indy.' Now, we played terrible and everybody's like, 'Well, what's going on?' Well, it's still a preseason game. Do you not feel the same way? It's a preseason game, you don't want to play that bad obviously, but we did. We know what we need to correct, and we'll do it. Come the Indy game, you won't see that defense, I'll tell you.”