Just when things were looking up for the 49ers, they lose their most productive defensive player.

Just when things were looking up for the 49ers, they lose their most productive defensive player.

Middle linebacker Derek Smith, who led the Niners in tackles each of their first three games, tore the medial collateral ligament in his left knee in a collision with a teammate during a pass play at Wednesday's afternoon practice.

A Magnetic Resonance Imaging exam revealed no further damage to Smith's knee, and Niners coach Steve Mariucci said Thursday afternoon that the injury won't require surgery. Smith - a key factor in San Francisco's 2-1 start - is out for Sunday night's game against the Carolina Panthers, and could miss between two and five weeks.

The 49ers have a bye week after their Oct. 14 game in Atlanta, and the team is aiming for Smith to return Oct. 28 against the Bears in Chicago. Mariucci, however, said, "I'm not going to eliminate him yet," from possibly playing next week against the Falcons. "He's a quick healer," Mariucci said. "Not quick enough for this (Carolina) game, but he'll be back soon. It's a week-to-week thing. He won't play this week. We'll see what happens next week."

Rookie Jamie Winborn will take Smith's place on the field - but not Smith's place in the lineup. Winborn, San Francisco's second-round draft pick this year, will start at the right inside linebacker position where Jeff Ulbrich started the Niners' first three games. Ulbrich will move into Smith's left inside linebacker position. Winborn began training camp as the starter at right inside linebacker with Ulbrich backing up both inside positions. But Winborn injured a knee late in the preseason, giving Ulbrich an opportunity to start. Ulbrich has played well and held the job since.

After missing San Francisco's first two games, Winborn made his NFL debut in Monday's win over the New York Jets. But he played primarily on special teams and saw only two plays of action on defense.

"I'm just going to go out and do what I do, and that's play football," Winborn said. "I'm excited to play like I was before I got hurt. I think this time around I should be better because I've had the chance to watch and learn while I was out."

Smith will be a difficult player to replace, however. Roaming from sideline to sideline, he had 37 tackles in San Francisco's first three games, and also excelled in pass coverage. Smith, who came to the Niners as a free agent in March after spending his first four NFL seasons in Washington, had provided an immediate impact and substantial boost to a defense that ranked 29th in the league last year. "We just keep going," Niners defensive coordinator Jim Mora said. "Obviously, you're going to miss a guy like Derek. He was very solid for us. But everyone will pick up the slack, and Jamie will go in and do a great job."

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