Dunne was dismissed for two bookable offences in the 3-1 defeat at relegation rivals Tranmere last week, which ended the Grecians' nine-match unbeaten spell and he must now serve a one-match ban, meaning City boss Paul Tisdale will be forced to change his starting line-up.
City also wait on the fitness of veteran striker Marcus Stewart (hamstring) after he went off at half-time at Prenton Park.
His place went to on-loan Rotherham striker Ryan Taylor, who could be poised for a start if Stewart fails to prove his fitness.
City are two points and a place above fourth-bottom Tranmere with three games remaining.
