The striker was signed from Charlton Athletic in January.
Walsh told the Sunday Times: "I saw him against Oldham Athletic for Charlton, and never changed my mind on him.
"He's got two feet. He's not one of those people who shifts it to his stronger foot to finish. He has an eye for goal. When he came on (against City) and scored the goal that took him into the fans' hearts. He looks to get his shots off, he does things that are unusual, he tries to find a yard with his first touch. Has a little pace.
"There are things we'd like him to add to his game but he's a young kid who has stepped up two leagues and so it'll take him time.
"It was a coup to get him here from London, but he didn't think twice – he looked at the pathway. He thought, 'I could get in that first team,' and he was right. I really wanted the player and the Chairman backed my judgement. To do that for a teenager is great."