The Ravens said he had "significant neck pain" but movement in his arms and legs. He was released from the hospital on Monday.
McGahee caught a pass and took two steps before Clark stopped him with a helmet-to-helmet hit. McGahee's neck snapped back and he immediately dropped to the ground.
He lay motionless for several minutes before being lifted onto a stretcher and taken away on a cart. McGahee clasped hands with teammate Ed Reed before he was taken off the field.
The play occurred with 3:29 left in Pittsburgh's 23-14 victory.
"It was a heck of a hit, for one thing," Baltimore coach John Harbaugh said. "Your first thought is obviously concern for Willis. Once we got out there, and hearing the doctors talk and see him move around, you felt like it was going to be OK. Both sides obviously had concerns, but we were happy it was not immediately serious at that point."
Dozens of players gathered around McGahee and Clark, both of whom were floored on the play.
"We hope that Willis is OK," Steelers quarterback Ben Roethlisberger said. "We're keeping him in our prayers. I know how scary that is. I started getting a little emotional when I saw him down there and the cart was coming out. That's the nature of this rivalry."
McGahee scored both Baltimore touchdowns and finished with 60 yards rushing, most by anyone on either team.
The hit also left Clark groggy. He had a dazed look as he wobbled off the field, his arms draped around a pair of Steelers staffers.