That's because the news could have been much worse for Aubrayo Franklin, the unlikely centerpiece of Nolan's three-year scheme to install a true 3-4 defense in San Francisco.

Franklin left training camp on a cart during the morning practice with a knee injury, but an afternoon MRI exam revealed only a sprained medial collateral ligament. The 49ers expect Franklin, who left Baltimore to sign with San Francisco in the offseason, to be ready to fill his spot in the center of the 49ers' defensive line by early September.

"It's not as bad as it could have been," Nolan said. "Missing training camp doesn't get me excited. ... That's a disappointment, but it's not that bad."

Franklin was injured when two offensive linemen rolled onto his leg during a bootleg play. He fell facedown, then rolled over holding his left knee while trainers and Nolan huddled around him, fearing the worst. Franklin already had strained his left elbow during the first week of training camp, missing some workouts.

While the 49ers made a dramatic mark on the offseason free-agent market with their signings of cornerback Nate Clements and safety Michael Lewis, their acquisition of Franklin was just as important to Nolan's plans to improve a defense that struggled horribly in the first two years of his tenure.

Nolan, a longtime defensive coordinator, fire#?# scheme in 2007.

The club alternated among various defensive fronts in Nolan's first two years, but Nolan never had the personnel to run a true 3-4 scheme before. San Francisco chiefly lacked a top-notch nose tackle, struggling to fill the spot with former University of Hawaii lineman Isaac Sopoaga, Ronald Fields, Anthony Adams and other unsuited linemen last season.

And though Franklin had just one sack and 40 tackles during his first four NFL seasons with the Ravens, Nolan — Baltimore's defensive coordinator in Franklin's first two years — has big plans for the Tennessee product. The 49ers' coaches see him as the prototypical run-stopping, pocket-pushing nose tackle who's necessary for the success of any 3-4 defense.

"He's not a finesse player," Nolan said of Franklin. "He gives us what we need. In a 3-4 defense, it's great to have a nose (tackle) that can do it all. ... His strengths lie more in pushing the pocket in pass (defense) and playing the run. He's a very good run defender, very good technician."

With a top nose tackle clogging the middle of the line, the 49ers' four linebackers are free to anticipate plays. Rookie Patrick Willis already is excelling in pass coverage during camp, while Manny Lawson, Brandon Moore and Derek Smith also will be more effective with a space-clogging lineman in front of them.

But Franklin, who didn't speak to reporters after getting hurt, must be healthy to make it all possible.

Sopoaga and rookie Joe Cohen will get the first chance to replace Franklin, while Fields could move over from defensive end to help out. The 49ers have high hopes for Cohen, a 310-pound fourth-round pick who was a stalwart on Florida's defensive line.