After two years Borgonovo was sent on loan again, this time to ACF Fiorentina, where he had the most successful season of his career, scoring 14 league goals in 30 appearances while playing alongside attacking partner Roberto Baggio under coach Sergio Santarini. The two forwards would form an important friendship, and their prolific goalscoring exploits that season (together they scored 29 of Fiorentina's 44 Serie A goals that season) would earn the attacking pair the nickname "B2", as both their surnames began with the letter B; together, they helped Fiorentina to qualify for the UEFA Cup. One of his most memorable performances with Fiorentina came in a 4–3 win over Giovanni Trapattoni's Inter Milan side on 12 February 1989, in the last match of the first round; Inter were in first place, and were unbeaten in the league at the time. With Fiorentina trailing 3–2, Borgonovo first scored a goal to tie the match, and subsequently also netted the match-winner in the 85th minute, after hiding behind Inter defender Riccardo Ferri, and suddenly running to intercept Giuseppe Bergomi's back-pass, who later admitted that he was not aware of Borgonovo's position; Borgonovo subsequently anticipated Inter goalkeeper Walter Zenga's run and took it past him before putting the ball into the empty net to win the match. Inter went on to win the next eight matches and eventually won the Serie A title that season, setting a new record for most points in a single Serie A season; the team went on a second unbeaten streak, which was interrupted by a defeat to Torino in the penultimate match of the season.