Zimmer told reporters that Bradford is working his way with the other quarterbacks but "has a ways to go" after completing his fifth practice since returning last week while competing for practice time with Teddy Bridgewater.
"It's kind of hard to get reps when you're getting your starter ready," Zimmer said the team's practice facility in Eden Prarie, Minn.
Bradford underwent knee surgery on Nov. 7 and was placed on injured reserve.
Bradford has played six quarters for the Vikings this season, and his last action came in the first half of the Oct. 9 victory over the Chicago Bears, when he was sacked four times before being replaced in the second half.
The Vikings are dealing with a three-week window to place Bradford on their active roster or keep him on injured reserve. If they win the NFC title, the latest they could make a move is Jan. 24 -- three days after the NFC Championship Game.