The former Chelsea defender picked up a hamstring injury during Forest’s Championship game against Queens Park Rangers away from home on Saturday afternoon.
The 27-year-old suffered the injury in the opening minutes of the second half at Loftus Road and was immediately substituted.
After the game, Freedman gave his thoughts on the injury suffered by Mancienne and told Forest’s official website: “It was a bitter blow for us, it really was. But I think we’re used to blows at this football club - we know how to handle them.
“I don’t know exactly how long he’ll be but with hamstrings it can be anything from two-to-six weeks. Let’s hope it’s the first one.”
Over the course of the 90 minutes at Loftus Road, the Reds had 52% of the possession, took 21 shots of which seven were on target, and won four corners.