As such, the Steelers should be awfully worried right about now.
In the first quarter of today's game against the Cleveland Browns, Mendenhall injured his right knee at the end of a run. Per Pro Football Talk, it was obvious right away that Mendenhall was in pain, and he had to limp off the field.
Officially, the word from Mary Kay Cabot of The Plain-Dealer is that Mendenhall's return is questionable. The Steelers are currently nursing a 13-9 lead in the fourth quarter.
The bigger concern for the Steelers right now is whether or not Mendenhall will be ready to return for the playoffs. With the Baltimore Ravens leading the Cincinnati Bengals, it looks like the Steelers will have to settle for a wild-card berth instead of the AFC North title.
That means they will not have a first-round bye, and that means Mendenhall will have just a week to get healthy.
The Steelers offense has enjoyed plenty of success through the air this season, but Mendenhall is by far their best running back. To date, he has 928 yards rushing on the season, making him Pittsburgh's leading rusher by a wide margin.
If Mendenhall has to miss time or is in any way limited, there's going to be a lot of pressure on Isaac Redman and rookie John Clay, neither of whom are proven NFL running backs.
By extension, there will also be a lot of pressure on Ben Roethlisberger, who suffered a nasty high ankle sprain early in December.
The Steelers can certainly win without Mendenhall if he does indeed have to miss time. Nevertheless, winning will be more difficult without him.