Duff has had a scan on his calf problem and is currently waiting for the results, on-loan Oliver Bozanic has returned to parent club Reading for treatment on knee ligament damage and fellow midfielder Michael Pook is still around two weeks away with a similar problem.
Striker Elvis Hammond has damaged ankle ligaments and - although the problem is not as bad as first feared - he will not be available this weekend.
Defender Drissa Diallo is rated 50/50 as he looks to get over an ankle injury of his own.
A new-look back four is expected to take to the field at the Darlington Arena as a result of the club's mounting injury crisis, but manager Martin Allen will be able to call on midfielder Frankie Artus after the 20-year-old returned to Whaddon Road on a one-month loan deal from Bristol City.
Ajet Shehu is not ready for involvement, however, as he lacks match fitness following his arrival on a non-contract basis. The Kosovo-born defender was released by Tottenham at the end of last season.