Tom Clarke nodded his first North End goal since his summer arrival in the 87th minute to send Grayson's side through to round two at the expense of his old club.
But at the final whistle, hundreds of home fans swamped the pitch, prompting police horses to enter the fray and disperse the unsavoury end to the Lancashire derby. A steward appeared to be trampled on by a police horse following the invasion of the Deepdale pitch as he helped to clear it of fans.
The steward, wearing a hi-vis jacket, looked to have been struck by the horse as it cantered across the pitch, while another horse - also helping with crowd dispersal - seemed to clip him as he lay on the ground. Lancashire Police confirmed they were looking into the incident.
"You don't want people to be running on the pitch and I would have preferred them to stay behind as the football club might get in trouble over what's happened, but hopefully there's not been too many people seriously injured," Grayson said.
"Hopefully it was only supporters being boisterous and there was nothing too intense or damage done to anybody as you see how much it meant to everybody at the football club.
"The result shows everybody what it means to the football club. I thought it was a cracking cup tie with a great atmosphere with some good football and good defending.
"I thought we played some good football, with great shape and organisation and worked hard, passing the ball when we could in what was a tight game."