Strong safety Rodney Harrison, the heart of the New England Patriots' secondary and long regarded as one of the league's most aggressive players at the position, will miss the rest of the season with a torn anterior cruciate ligament in his left knee. Three sources confirmed the severity of the knee injury, suffered in the first quarter of Sunday's 23-20 victory over the Pittsburgh Steelers. There could be ancillary damage beyond the ACL as well. The team almost certainly will place Harrison, 32, on the injured reserve list. At his regular Monday news conference, coach Bill Belichick said the status of injured players would be updated Wednesday, when the team is compelled by league policy to report players' availability. New England hosts San Diego, the team with which Harrison played the first nine seasons of his career, on Sunday afternoon. Harrison's injury is the latest setback to a New England secondary already battered at the cornerback position. This will mark the second consecutive season in which the Patriots will be forced to overcome injuries in the secondary and in which the team's mettle and ability to compensate for the loss of key players will be tested. The loss of Harrison, coupled with the absence of inside linebacker Tedy Bruschi, leaves New England without two of its smartest and most inspirational defenders as the Patriots attempt to become the first franchise in history to claim three straight Super Bowl titles. In addition to Harrison, the Patriots likely will be without starting left offensive tackle Matt Light for the foreseeable future. Light suffered an injury to his right knee but full details were not yet available. Second-year safety Guss Scott, who replaced Harrison for the final three quarters Sunday, is the candidate most likely to move into the starting lineup. Scott was a third-round pick in the 2004 draft but, ironically, missed his entire rookie season because of a knee injury. The Patriots may explore alternatives outside their organization but there are not many quality safeties without jobs. Harrison, a multiple Pro Bowl performer, is regarded by some in the New England organization as one of the most irreplaceable players on the roster. Beyond his playmaking skills, and ability to get everyone aligned in the right spot, his on- and off-field leadership will be sorely missed. The 12th-year veteran was injured about midway through the first quarter when Steelers wide receiver Cedrick Wilson fell onto his leg during a running play. Harrison's knee bent grotesquely. He was on crutches, and wearing a brace, when he left Heinz Field. Harrison is in his third season with New England, having signed with the Patriots in 2003, after the San Diego Chargers released him for salary cap considerations. In 158 games, Harrison has 1,027 tackles, 27½ sacks, 31 interceptions and 104 passes defensed.