Wigan had a number of chances to have won the game themselves before Harper netted and in the first half saw referee Keith Stroud award them a penalty which he then downgraded to a free-kick.
Coppell admitted: "It was a massive goal for us and came at a stage in the game when if anyone was going to win it was them. We were affected by their goal. In the first half we played well but when we scored we lost a bit of an edge.
"In the second half once again we couldn't go five minutes without conceding. They seemed to grow with the goal and on a couple of occasions we had to defend with our life's worth.
"Then to score at the end, we have had a few things go against us so it was nice to have that break coming our way.
"It was desperately important to win and there is no use hiding it. I said beforehand that it would be psychologically important and it's not how you win a game, just the fact that you have won it - and we have won it. Not in a convincing fashion but we have won it and we are out of the bottom three."