CHICAGO – Kyle Orton delivered a strong performance in his first game since being named Bears starting quarterback. But a shaky outing by Chicago’s defense in Thursday night’s 37-30 preseason loss to the San Francisco 49ers created more questions than answers.

Orton guided the Bears (0-3) to two touchdowns and two field goals on their first four possessions. He completed 10 of 17 passes for 147 yards with 2 TDs, no interceptions and a 126.3 passer rating before being replaced by Rex Grossman with 9:21 remaining in the third quarter.

Rashied Davis hauls in one of his two TD receptions from Kyle Orton in Thursday's loss to the 49ers.

But the Chicago defense played poorly from the outset, allowing the 49ers (2-1) to score two touchdowns and two field goals on their first four drives. San Francisco compiled 248 total yards in the first half, including pass plays of 41, 40 and 37 yards and a 28-yard run.

“On the defensive side of the football, I was just completely disappointed with the entire effort,” said coach Lovie Smith. “We’re a better defense than that. I thought after last week we would get that swagger back and come out strong. Every indication I got this week said that we would do that, but we didn’t get it done. We’ll go back, make the corrections and try to go from there.”

The 49ers offense ranked last in the NFL in 2007, averaging 237.3 yards per game. But on Thursday night, San Francisco rushed for 160 yards on 35 carries and passed for 269 yards.

J.T. O’Sullivan, who has had two stints with the Bears, moved closer to securing the 49ers’ starting quarterback position by completing 7 of 8 passes for 126 yards with 1 touchdown and a perfect 158.3 passer rating before leaving the game early in the second quarter.

“If we play like we did tonight, we’re not going to win many football games,” said safety Mike Brown. “The offense played great. If they put up that many points, we should come away with the win and we weren’t able to do that. San Francisco came out and dominated us from the first series on and there was really nothing we could do about it.”

Orton did his best to keep the Bears in the game, connecting with Rashied Davis on TD passes of 21 and 7 yards in the first half. The two came close to hooking up a third time, but Davis dropped Orton’s pass in the end zone on third-and-three from the 6 late in the second quarter.

“We certainly wanted to play well this game because we won’t be playing too much the next game,” Orton said after Bears starters saw their most extended action of the preseason.

“This was our best chance to find out where we were as an offense. We scored some points, but we left some plays out there on the field. It’s good to know that you can score some points and then still have some room to improve. Hopefully we can get better these next two weeks and be ready to go for Indianapolis [in the Sept. 7 season opener].”

After struggling last Saturday night in Seattle, the Bears offensive line had a stellar game, opening holes for Matt Forte (11 carries for 44 yards) and Kevin Jones (3-30) and not allowing Orton or Grossman to be sacked on 21 pass attempts.

Danieal Manning had kickoff returns of 75, 60 and 36 yards against the 49ers.

“I thought the guys up front played great,” Orton said. “We established the run game early and they gave me all day to pass.”

Danieal Manning set up the Bears’ first 10 points with kick returns of 60 and 75 yards in the first quarter. On his first return, he took a handoff from Devin Hester on a reverse, broke two tackles inside his own 5 and raced down the left sideline to the San Francisco 33. Manning fumbled at the end of his second return, but Kevin Payne recovered at the 49ers’ 14.

The 49ers scored two TDs in a 2:50 span early in the second quarter to turn a 10-6 deficit into a 20-10 lead. On back-to-back plays, O’Sullivan lofted a 40-yard completion to tight end Vernon Davis and tossed a 37-yard TD pass to receiver Jason Hill in the right corner of the end zone.

Garrett Wolfe then lost a fumble on the ensuing kickoff, and San Francisco converted the turnover into Moran Norris’ 1-yard TD run.

Orton later directed scoring drives of 81 and 74 yards as the Bears erased the 10-point deficit on Davis’ second TD and Robbie Gould’s 24-yard field goal to forge a 20-20 halftime tie.

“I thought I played well,” Orton said. “I thought we played well as a unit. As a unit we’ve been playing all right, we just haven’t been scoring points and finally we kind of got over that and put up 20 points in the first half, which was good.”

The Bears’ starting defense exited after yielding Alex Smith’s 12-yard TD pass to receiver Dominique Zeigler, which gave the 49ers a 27-20 lead with 9:28 to play in the third quarter.

“We’re very disappointed in our play tonight,” said defensive coordinator Bob Babich. “We are a very good defense. We are going to be a dominant defense. We just need to make sure when we go out and play we play at that level.”

Gould’s 28-yard field goal cut the gap to 27-23 midway through the third quarter. The kick came after Brandon Lloyd blocked a 49ers punt, giving the Bears the ball at the San Francisco 18.

After Thomas Clayton’s 15-yard TD run and Ricky Schmitt’s 31-yard field goal increased the margin to 37-23, Brandon Rideau outleaped a defender to haul in Caleb Hanie’s Hail Mary pass on the game’s final play for a 51-yard touchdown.