Mendenhall sustained the injury late in the first quarter while getting tackled following a 5-yard gain. The team's leading rusher clutched the knee immediately and had to be helped off the field. He was on crutches following the game and coach Mike Tomlin indicated Mendenhall would at least miss this weekend's Wild Card game against Denver.
An MRI on Monday revealed the extent of the damage, leaving third-year running back Isaac Redman and rookie John Clay as the only healthy backs on the roster. Veteran Mewelde Moore is nursing a sprained left knee and Jonathan Dwyer is already out for the season with a left foot injury.
"It's something that's tough but I definitely wish him the best and he'll be with us regardless," tackle Max Starks said of Mendenhall.
A budding star after rushing for over 1,100 yards in 2009 and 2010, Mendenhall finished with 928 this season as the offense relied more heavily on quarterback Ben Roethlisberger.
Still, Mendenhall was a reliable goal-line option, scoring nine touchdowns behind a patchwork offensive line that spent the first half of the season getting shuffled around each week.
Redman lacks Mendenhall's breakaway speed but can be effective in between the tackles. He ran for 92 yards and a touchdown against the Browns after Mendenhall went down.
"He's a tremendous downhill rusher and at one point I think the Cleveland Browns were arguing among themselves about who was going to tackle him because they couldn't get him down," Starks said. "He's a hard runner and I'm looking forward to seeing him featured."
Just as long as Redman holds onto the ball. He fumbled twice on Sunday, helping Cleveland keep things interesting until the final seconds.
"It's one of those things like, 'Hey, hold onto the ball tighter,' " Starks said. "Still to come in and get as many plays as he did, not expecting to get that many run plays I think he did admirably. Now that we've got (the fumbles) out of the way, we tell him 'You're the feature guy, you need to prepare for it.' "
If Redman stumbles the Steelers will turn to Clay, a practice squad player most of the season who made his debut in a 24-0 win over St. Louis on Christmas Eve. The former Wisconsin star ran nine times for 31 yards while spelling Redman in the second half against the Browns.
Whoever is back there will need to run effectively to take some of the pressure off Roethlisberger, who continues to deal with a severely sprained left ankle. He completed 23 of 40 passes for 221 yards in Cleveland and will face one of the league's most aggressive defenses in the Broncos, who finished fourth in the AFC with 41 sacks.
The Steelers are hardly intimidated by the prospect of having to play three road games to reach their fourth Super Bowl in seven seasons. They won at Cincinnati, Denver and Indianapolis in the 2005 playoffs before knocking off Seattle to capture their fifth Lombardi Trophy.
The New York Giants in 2007 and the Green Bay Packers last year took the same route to a championship and the memories of Pittsburgh's run in '05 remain fresh for the 16 players off that team still around, including Roethlisberger, safety Troy Polamalu, wide receiver Hines Ward and linebackers James Harrison and James Farrior.
"The guys that have been here, we understand what it takes," Farrior said. "It's not anything new to us. We've been in that situation before and we reached the mountaintop."
To do it again Pittsburgh will need to find a way to play better on the road. The Steelers went 5-3 away from Heinz Field this season, with all three losses to playoff teams -- Baltimore, San Francisco and Houston -- in games that weren't particularly close.
The Ravens drummed the defending AFC champions 35-7 in the season opener, Houston won 17-10 on Oct. 2 and San Francisco took advantage of a hobbled Roethlisberger in a 20-3 win two weeks ago.
Pittsburgh's lone road victory against a 2011 playoff team came in a 24-17 victory at Cincinnati.
Farrior acknowledged the Steelers haven't exactly been sharp on the road against quality teams. All that means is now is a good time to start.
"We love to have our backs against the wall," he said. "We're going to have to go on the road. We're going to have an 'us against the world' mentality. It's a little overrated but it works sometimes."
Information from ESPN NFL Insider Adam Schefter and The Associated Press was used in this report.