Raynes, 24, was carried off on a stretcher during last Saturday's 1-1 draw with Northampton.
The Millers initially thought he had damaged muscle in a cruciate knee ligament, but rather than being out for much of the rest of the season he could be back in little over a week.
"We think the actual knee itself is okay but he has a problem with his calf and Achilles so we think it is a case of nerve damage rather than muscle damage,'" manager Andy Scott said.
"We are glad that it is not a major knee problem which we did in the first case. We are probably looking at a week before he is fit.'"