Cooper featured for Leeds in the Championship game against Brighton at Elland Road on Saturday afternoon.
The 25-year-old, who can operate as a centre-back or as a left-back, started the match and played for the entire 90 minutes.
The Scotsman replaced Pontus Jansson in the starting lineup, and he has revealed when he knew about Garry Monk’s decision.
“The gaffer told me (on Friday) that I was playing and I was obviously buzzing because I’ve waited a long time,” Cooper told The Yorkshire Evening Post.
“I have been professional and always will be but it’s been hard as I haven’t played as much as I’ve wanted to. The gaffer has shown confidence in me and I’d like to think I’ve repaid him.
“Pontus and Barts have been unbelievable and you can’t knock that. We’ve had a lot of clean sheets this year. It is hard coming in as you’re not up to top speed but you can’t moan about that. You have to go and do a job.”
Cooper played well for Leeds, who kept a clean sheet against Brighton and won the Championship match 2-0 to maintain their quest to finish in the top six this season.
The former Chesterfield defender has been called up to the recent Scotland team, and that is only going to boost his confidence.
Pontus Jansson, who is very popular among the Elland Road faithful, should return to the starting lineup soon, but the Leeds fans should be encouraged by Cooper’s performance against Brighton.