Mark participated fully in Tuesday's practice, working both at running back and on punt returns, where he earned All-America honors last year after averaging 18.7 yards per return.
"It's a heck of a free-agent pickup," Northwestern coach Pat Fitzgerald said. "You get an All-American back four games into the season."
Mark said he sustained the injury two days into preseason camp. He wasn't 100 percent for the opener against California but played and logged 11 carries and one reception but did not handle returns.
He then sat out games against Syracuse, Western Michigan and Maine before returning to practice last week. Northwestern hasn't specified the injury, and Mark, when asked Tuesday if it's a hamstring, smiled and called it simply a lower-body issue.
"I feel real good now," said Mark, who rushed for 1,366 yards and 12 touchdowns on 226 carries last season, earning second-team All-Big Ten honors. "I'm just ready to help the team. I've been itching to get back on the field. I feel like a spectator a little bit. I'm not really used to that at all.
"I'm ready to go."
Mark, who likely will handle punt returns but not kickoff returns Saturday for the 16th-ranked Wildcats (4-0, 0-0 Big Ten), called the opportunity to play against Ohio State (5-0, 1-0) "a reward."
"Every time they step on that field, they're ready to play," Mark said. "We know that. We're also ready to play. We have a lot of confidence right now. They're coming into our house, and we definitely expect to defend our house."