Mendenhall, the team's leading rusher for the past three seasons, tore his anterior cruciate ligament in the Jan. 1 regular-season finale at Cleveland.
"I never feel good about an ACL for a year," Colbert told Pittsburgh reporters Monday.
Colbert didn't describe running back as a pressing need heading into free agency, which begins March 13.
"No more than we are for any other position," he said, when asked if the team needs to add another running back. "Our young backup contributors did a nice job."
Colbert also indicated that the Steelers haven't parted ways with wide receiver Hines Ward. The NFL Network on Saturday reported that the Steelers had decided not to bring back Ward.
Ward, who lost his starting job in 2011, was scheduled to make $4 million in the last two years of his contract but has said he was willing to take a pay cut.
"To this current point no final decisions have been made," Colbert said. "We've had several discussions internally."
Colbert said the team is in the middle of making free agent evaluations and won't make any move on Ward until that process is complete.
"That will give us an idea about what else is out there, where we stand with restructuring contracts and what we will have to do as far as possibly terminating people, both immediately and after the fact if we decide we want to add some players (in free agency)," Colbert said.
Ward is coming off a season in which he finished with 46 receptions for 381 yards and two touchdowns, his lowest totals in each category since he was a rookie in 1998. The 2006 Super Bowl MVP -- who also won "Dancing With the Stars" last spring -- is one of only two receivers in league history to reach 1,000 receptions and win at least two Super Bowls.
Jamison Hensley covers the AFC North for ESPN.com. Information from The Associated Press contributed to this report.