The Robins skipper has been missing since the dramatic 5-4 defeat to Bradford on August 22 when he was forced to hobble off with a muscle pull at the bottom of one his calves.
His latest setback comes after an Achilles problem that severely restricted his involvement last term, but Duff is hopeful the two are not related.
"It's more muscular, which is a good thing," he told the Gloucestershire Echo. "We are not sure how long it is going to take to settle down, but I am going to stay off it for a couple of weeks and see how it goes."
Duff is just one of four long-term absentees within Martin Allen's ranks but, overall, the signs are looking positive in the treatment room for the League Two outfit.
Elvis Hammond (ankle) is expected to return to light training this week and could feature on Saturday at Bury, although the game is likely to come a little too soon. Michael Pook (knee) is looking at two weeks while loanee Oliver Bozanic (knee) should be back in action in around a month.