The Manchester United forward only arrived on loan at St Andrew's on Thursday but required little time to settle in as he immediately found his scoring boots, netting a 13-minute first-half hat-trick.
It came on what was his first full start in professional football having not yet featured for United and managed just five substitute appearances during a loan stint at Leicester last term.
"I enjoyed it. It's a dream come true, really," he told Birmingham's official website. "After the first goal my confidence was up and I just wanted to score more goals then.
"I'm glad we got the three points as well in the end which is massive for this club. I know they've haven't got off to the best but obviously I have come here to change that and scoring the goals has got our confidence up.
"I couldn't carry on because I had cramp in the second half. Hopefully I can play against Swansea on Wednesday but if not I'll be looking forward to Reading next weekend."
Manchester United are yet to agree if he can feature for Blues in the Capital One Cup.