In 1973, he was signed by Serie A club Fiorentina, where he emerged as a young but promising attacking midfielder and won the 1975 Coppa Italia thanks to a goal of his in the final against Milan.

Despite being only aged 22, Guerini immediately entered into a non-playing career and became youth coach at his last club Fiorentina, also winning a Campionato Nazionale Primavera title in 1983.

In 1985, he was then hired by another Tuscan club, Pisa, thus having the chance to make his coaching debut in the Serie A league; despite winning a Mitropa Cup on that same year, Guerini however failed to save the team from relegation.

He then served as head coach of Catanzaro, narrowly missing a top flight promotion, and successively Brescia.

On 2 May 2012, he was then appointed caretaker coach after the club decided to remove Delio Rossi from his managerial duties following a physical confrontation with the Serbian player Adem Ljajić during the home league game against the Novara.