"There's no doubt he's one of our better performers on defense throughout this season, but ... we've lost some pretty good performers throughout the season and this team's been resilient," coach John Fox said.
Harris was injured in the third quarter of Denver's 24-17 win over San Diego in the AFC Divisional round Sunday.
After he went out, Chargers quarterback Philip Rivers staged a comeback from a 17-point deficit largely by targeting Harris' rusty replacement, veteran Quentin Jammer. It fell short, however, when Peyton Manning was able to keep Rivers on the sideline over the final 3 minutes, 51 seconds by converting a trio of third downs.
Harris had played more snaps than anybody on defense and had 65 tackles, three interceptions and 15 pass breakups, all career highs.
"It's another disappointment for us," linebacker Paris Lenon said. "Somebody else that we've been battling with that has been a huge part of this team and this defense that is down. It's an opportunity for somebody else to step up and play big."
Fox wouldn't say if Jammer will start against New England in the AFC championship.
The Broncos (14-3) have endured an injury epidemic, especially on defense, to reach the AFC title game against New England (13-4), but Harris' injury could prove the hardest to overcome.
"It's terrible news," Terrance Knighton said. "You hate to see a guy having a great year, going into a contract year (get hurt). I wish him the best. I wish him a speedy recovery, but we just have to focus on the task at hand right now and that's New England. Whoever will fill his shoes, whoever we put in there, has to step up."
If the Broncos don't stick with Jammer, they could move Champ Bailey, a 12-time Pro Bowler who's been relegated to slot duty after returning from a nagging foot injury last month, back outside or they could insert rookie Kayvon Webster in Harris' spot opposite shut-down cornerback Dominique Rodgers-Cromartie.
Webster is playing with a cast on his right thumb, which he broke in two places a month ago, requiring surgery to insert six screws and five pins.
Another option is Tony Carter, who was covering a punt at New England on Nov. 24 when returner Wes Welker didn't call him off in time and the ball hit his leg. The Patriots recovered and Stephen Gostkowski kicked a 31-yard field goal that gave New England a 34-31 win over Denver in overtime.
Harris is the fifth defensive starter the Broncos have lost, joining Von Miller, Kevin Vickerson, Rahim Moore and Derek Wolfe.
Harris was hurt on the opposite side of the field when Rivers completed a 19-yard pass to Keenan Allen along the San Diego sideline with 8:17 remaining in the third quarter.
Harris helped hold Rivers to 20 yards passing in the first half but after he went out, Rivers threw two TD passes to Allen and it took Manning's 21-yard completion to tight end Julius Thomas on third-and-17 from his own 20 to help the Broncos salt away the win.
"Philip got hot there in the second half. I did not want to give him the ball back there at the end," Manning said. "There was some real want-to on our offense's part to stay on the field on that last series."
Harris, who received a $2,000 signing bonus as an undrafted free agent out of Kansas three years ago, is set to become a restricted free agent this offseason and was certain to receive the highest possible tender from the Broncos, which figures to be in the $3 million range.
A torn ACL typically requires six to nine months of recovery and rehab.
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