The full-back lasted just 11 minutes of the impressive 3-1 win at Charlton on Saturday before going off after injuring himself in an aerial challenge, so Liam Sercombe is in line to start at Brunton Park.
Sercombe replaced Duffy at the Valley and scored the opener with a powerful finish from the edge of the area.
Midfielder Jake Thompson has missed the last two games with a knock picked up in the Johnstone's Paint Trophy Southern area final defeat to Brentford and is a doubt again to feature.
Paul Tisdale's side could climb into the top 10 if they pick up a win from the near 700-mile round trip.
Source: PA
Source: PA