I don't think it was so much the signing of Trevor Francis. It was his debut the following Saturday at Stoke and his opening goal after two minutes.
Don't forget that Francis had scored the winning goal in the European Cup Final only two years earlier and was the best player in Engfland at the time.
Somebody mentioned that they can't remember Prter Beardsley playing for City. How about Martin ONeill, Ken Mcnaught or David Johnson. All of them won the European Cup before joining City.