The Lilywhites boss believes that his side deserved to win the fourth-round contest, describing the result as "daylight robbery" in his post-match interview.
"We said to the players before they went out 'do yourself justice, do the club justice and put on a performance' and we certainly have done that tonight," BBC Sport quotes him as saying.
"Everybody saw the commitment, the effort, the quality. To limit such a strong, powerful team as Nottingham Forest to the number of chances they had and to create the chances we did.
"It was an understatement that it was daylight robbery today."
Forest will take on Sheffield United in round five.