Lawson suffered the knee injury late in practice Wednesday and a subsequent MRI revaled the tear. He will require season-ending surgery and will eventually be placed on the injured reserve list.
"I heard a pop, and I remember what [teammate] C.J. Brewer had talked about when he [injured his knee], so I was nervous," Lawson told the team's Web site. "I tried to test it out, but really I just had to wait for the MRI.
"I was really looking forward to doing big things this year for my team. The hardest thing is just knowing I won't be able to be out there practicing and playing in games with the guys."
Lawson was a first-round draft pick in 2006 who signed a five-year deal worth a maximum value of $10.14 million. In 16 games as a rookie, including 11 starts, Lawson had 57 tackles, 2½ sacks, one interception and two passes defensed.
This season, he was coming into his own this season as a pass-rusher.
His likely replacement is veteran Hannibal Navies in the 49ers' 3-4 defense. In addition to Navies, seldom-used second-year pro Parys Haralson is also a candidate to replace Lawson, though one
player probably can't fill all of Lawson's roles.
Coach Mike Nolan said he probably won't name a replacement for Lawson until game time Sunday in Pittsburgh.
Navies, a ninth-year pro with his fourth NFL team, started three
games for the 49ers late last season.
"I'm not just some guy that just got here and all of a sudden
needs to be playing," Navies said. "Obviously, I've been in there
playing a lot last year in the defense. I know the players, I know
the coaches, I know the scheme, so it's not uncommon."
Inside linebacker Brandon Moore also might have a chance to claim Lawson's spot. Moore was San Francisco's leading tackler last season, but has been a backup this year after standout rookie Patrick Willis claimed his starting spot during the preseason.
"It's still something that I've done in the past, something
that I'm comfortable with," Moore said. "Manny is an exceptional athlete. I know everyone around him had a lot of expectations for him. It's a setback for him, but Manny's the type of guy that will definitely rebound from this and come out on top."
Though San Francisco had eight linebackers on its roster after the club's conversion to a full-time 3-4 defense, the Niners' defensive depth already has taken a few hits. Jay Moore, a
fourth-round pick expected to provide depth at the position, was lost for the season with an ankle injury late in training camp.
"Obviously it's time for the backups to get an opportunity to
step up and do something," Nolan said. "We have four outside linebackers, so obviously it will come from one of the other two backups."
Nolan also wouldn't say how the 49ers plan to fill Lawson's roster spot after placing him on injured reserve. San Francisco could re-sign linebacker Roderick Green, who was among the club's final cuts in the preseason, or sign Mark Washington from the practice squad.
ESPN.com senior writers John Clayton and Len Pasquarelli contributed to this report. Information from The Associated Press was used in this report.