It was the Lilywhites' first loss in five outings - and only the third in 13 games under Grayson - but the former Leeds and Huddersfield boss has criticised his players for a lacklustre opening.
He said: "I'm disappointed because I didn't want us to go down with a whimper. We started very poorly, were not lively and didn't press them. We allowed them to dictate.
"In the second half we showed more energy, dominated and were more lively. A draw would have been a fair result, I think.
"We hit the bar through Jack King and could also have had a penalty as Nicky Wroe felt he was held back."
Preston end the season against Sheffield United at the weekend and Grayson wants his charges to bow out in a positive fashion.
"It will be a great match to be involved in and we want to go there and get a victory," he added.
"What we've done is give us a platform to build on for next season. I know what we need over the summer and hopefully we'll get it."