The Steelers picked Mendenhall for different reasons in 2008
Coming off the 2008 season, the Steelers roster was pretty well built. Using the archived depth chart from right after the draft, this team was pretty set. The defense was filled with talent, and the offense still had a productive core. The major weakness for the team was arguably the offensive line, as the group was aging and injury prone.
Instead of addressing the line with a top prospect such as Duane Brown, the team took Mendenhall. There wasn’t a need at running back on paper. Willie Parker was coming off his second consecutive Pro Bowl. The team lacked a valid power component to their running game though and instead of addressing the trenches, they took Mendenhall.
He was coming off a breakout season at Illinois, compiling 1999 total yards and 19 touchdowns. Not only was he big, but he also tested as a freak athlete. He had a RAS score of 8.95 despite being one of the thicker build-backs in that draft.
Despite bigger needs being elsewhere, the team took Mendenhall with the expectation that he would be the thunder to Parker's lighting. That didn’t materialize during his rookie year, as he was lost for the season after only playing four games. That said, Parker was beginning to regress, so it looked like the Mendenhall pick was going to pay dividends.