Posh's 2-1 win at London Road was the first victory under Cooper's management and the three points lifted them off the foot of the Championship table going into Christmas.
"I'm pleased, obviously," Cooper said after the match.
"First half I thought we caused them problems. Sometimes the best form of defence is attack.
"Second half, as much as we tried to guard against complacency it happened, we wobbled. That's got to stop, we've got to be mentally stronger.
"Once we regrouped we went on again.
"I'm relieved to get the first win.
"I hope it takes us into Christmas and the New Year with an added boost and a spring in our step."