Diamanti struck just before the hour as Bologna picked up only their third victory of the season. Pioli knew coming into the game that anything less than a victory would bring an end to his reign in charge of the Rossoblu. Before kickoff, Bologna had picked up just three draws from six matches, while Genoa were riding high in the top half of the table.
Bologna lined up without injured duo Frederik Sorensen and Nicolo Cherubin. Thomas Manfredini returned from suspension for Genoa, but Daniele Portanova, Francesco Lodi and Juraj Kucka missed the trip through injury. Alberto Gilardino faced his former club.
Bologna did much of the early running and had the best of a low key first half. Four minutes in, Cesare Natali nodded a free header wide from Diamanti's cross with Mattia Perin rooted to the spot.
Genoa's sole chance in the first period came on 19 minutes, but Gyorgy Garics cut out a low cross meant for Moussa Konate.
Davide Moscardelli cushioned a header down for Panagiotis Kone to volley into Perin's gloves on half an hour.
Six minutes before the break, Manfredini blocked Kone from connecting with a Diamanti cross. On the stroke of half time, Jonathan Cristaldo threaded through for Kone, but his weak cross was smothered by a grateful Perin.
Two minutes after the restart, Manfredini headed over a chipped cross from Sime Vrsaljko. Bologna then took a deserved lead. Diamanti collected a ball outside the area and placed a low shot beyond Perin which nestled nicely into the bottom corner. It was Diamanti's fourth strike of the season and no more than Bologna deserved.
Moscardelli almost doubled the advantage. His volley into the ground bounced back up, forcing Perin to tip the ball over the bar.
Gilardino succumbed to a shoulder injury and was given polite applause off the pitch by the Bologna fans. Diamanti picked up a costly late booking which will rule him out of Bologna's crunch game at Catania when Serie A resumes after the Christmas break. Genoa pressed late on, but they couldn't penetrate a solid Bologna defence.
Bologna: Curci; Garics, Natali, Mantovani; Kone, Perez, Krhin, Morleo; Diamanti; Moscardelli, (Crespo 85) Cristaldo (Acquafresca 76)
Genoa: Perin; Antonini, Gamberini, Manfredini; Vrsaljko, (Fetfatzidis 64) Biondini, Matuzalem, Bertolacci, (Cofie 56) Antonelli; Konate, Gilardino (Calaio 76)
Goals: Diamanti 57 (B)