All four corners of Sardegna Arena gave a tearful Ranieri a standing ovation before kick-off, but Fiorentina had no option but to try and spoil the send-off as they aimed to cement themselves in eighth and seal at least a UEFA Conference League place.
The visitors had the first big chance of the match in the 20th minute when Andrea Belotti powered a header on target as he connected with Cristiano Biraghi’s free-kick, only for Simone Scuffet to deny him with a spectacular save.
That spurred the Sardinian side into action, and they came within inches of breaking the deadlock multiple times in the next 15 minutes; Zito Luvembo was twice denied by goalkeeper Pietro Terracciano, who also kept an Adam Obert chance from looping in before Alessandro Deiola and Matteo Prati each saw efforts creep over the bar.
The hosts were made to pay for not taking one of those opportunities six minutes before HT when they afforded Giacomo Bonaventura too much space at the edge of the box, and the midfielder curled an effort past Scuffet to give La Viola the lead.
While he had no chance with the opener, Scuffet was showing off his reflexes again soon after, reacting well to a Gaetano Castrovilli strike. Having almost gone 2-0 down, Cagliari briefly thought they were going in level, but Gianluca Lapadula had a finish ruled out for offside.
Desperate to end their manager’s career on a high, 33 years after he had first guided Cagliari to Serie A survival, Gli Isolani began the second half well, with Lapadula going close again before Deiola sent a header wide from a corner.
Just after the hour mark, Deiola met Prati’s cross and guided the ball past Terracciano. The on-field decision was offside, but a VAR review overturned the call and the equaliser stood.
The 2015/16 Premier League winner thought he had a fairytale finish, as with five minutes remaining, Kingstone Mutandwa cut in from the left wing and curled a delightful effort from the edge of the area around Terracciano and in off the post – his first and only goal of the season.
But within three minutes, Fiorentina had restored parity when Nicolás González got free of his marker and steered a set piece in from close range.
From a party atmosphere after the Mutandwa goal, the local crowd was stunned into silence in the 100th minute when VAR spotted a foul in the area by Alessandro Di Pardo on Lucas Beltrán, resulting in a penalty.
Arthur Melo stepped up and scored the winner with the last kick of the game to break Cagliari hearts and secure UEFA Europa Conference League football for Fiorentina ahead of the final of the same competition with Olympiacos Piraeus next week.
Meanwhile, Cagliari sign off with a first home defeat in seven matches.
Flashscore Man of the Match: Alessandro Deiola (Cagliari)