A well taken first-half goal from midfielder Mike Jones proved enough to give the Reds victory.
It inflicted a fifth loss in the last seven outings on the U's and Dunne said: "It's not about tactics now, it's about guts, determination as an individual and as a player.
"We got a reaction in the second half at Crawley and now we must step up to the plate - it's about winning now.
"We can pick a team that can compete. It's about determination and to be ready to give everything on Tuesday."
Dunne admitted that his players "did not show up" in the first half at Brunton Park but felt they ultimately deserved something from the game after Sanchez Watt and substitute Dominic Vose were both denied by the woodwork.
He added: "Crawley went in front with a very well-worked goal and we had to have a chat at half-time.
"I wanted to see us play to our maximum and in the second half we got hold of the game, won more second balls and dominated more of the territory.
"We hit the post and the crossbar and deserved something from the game. But we must stay positive, it's not good being negative."