The 26-year-old made his first appearance of the season in Saturday's 2-1 win over Huddersfield due to an injury to Wayne Bridge.
Bridge, who joined on a season-long loan from Manchester City in the summer, limped off in last weekend's draw with Wolves and Painter says he is ready to seize the opportunity in his absence.
"I've got to admit I was nervous because it has been a long time since I started a game," he said. "It was different emotions to what I'm used to. I loved it. It was weird because it was like my debut again.
"The manager [Gus Poyet] said to me at the start of the season that Wayne would play but I had to be there to step in. He's been honest and said 'If you deserve to play, you'll play' - Wayne will play if he is fit and I accept that.
"I was aware Wayne came off last Saturday. I thought he wouldn't come off for the sake of it so I thought my opportunity could come."