The Millers number two spoke to the Advertiser today after his first game stood alongside gaffer Ronnie Moore.
Mullen said: "Promotion is never a gimme, no matter what level of football you're at. You have to earn the right to be in the position you need to be to get the rewards, which is obviously promotion through the play-offs or direct.
"The lads have got off to a great start and hopefully Tuesday is just a blip, which I'm sure it will be.
Mullen had checked Northampton out in their game against Rochdale when working as a scout.
"I saw Northampton a couple of weeks ago against Rochdale and although Rochdale won, Northampton were very good in the second half," he said.
"Apparently they didn't deserve to lose at Shrewsbury on Saturday either. "But apart from picking the ball out of the back of the net, Andy Warrington didn't have much to do on Tuesday.
"The first goal came from nowhere; it was a shot out of the blue. The second one struck the underside of the bar and bounced down over the line the third one could have gone anywhere but hit Ian Sharps on the head and flew in".
It was a busy few days for Mullen, having attending his sons wedding in Italy a couple of days before the Northampton game
"It was something I couldn't get out of. It's been planned 12 months," he said. "I spoke to Ronnie before going and he said he wanted me to come on board with him and would I get over and see him.
"I did that on Monday and met the chairman and the directors and was very impressed. "Tony Stewart is a very ambitious guy and I think he has the right man in place in Ronnie."
|Welcome to MillersMad|
Here's our LATEST Messageboard topics