Inter were forced to play the final 30 minutes a man down after Brazilian midfielder Felipe Melo was sent off, but the visitors were gifted all three points as a defensive error enabled under-fire striker Mauro Icardi to slot home the only goal of the game on 67 minutes.
Inter had been the early-season pacesetters after starting the season with five straight wins but a heavy home defeat to Fiorentina last month was the catalyst for a sharp decline in fortunes.
Since that defeat, Inter had managed only four goals in their last five matches, and much had been written about the slump that Icardi was in, and whether Icardi and summer signing Stevan Jovetic could work together.
For the trip to the Stadio Renato Dall’Ara, Inter boss Roberto Mancini dropped Jovetic and gave Adem Ljajic just his second start of the season as Inter sought a solution to their goalscoring troubles, but it was their defence that was almost breached five minutes in as Amadou Diawara’s searching ball just evaded the outstretched leg of Anthony Mounier.
Ljajic’s long-range free-kick that was easily held by Bologna goalkeeper Angelo Da Costa just before the break represented Inter’s only shot on goal in a quiet opening 45 minutes.
Fortunately the second half was better as LIvan Perisic, on target in Saturday’s 1-1 draw at Palermo, should have fired Inter ahead on 59 minutes as Icardi’s low cross broke kindly for the Croat, only for his tame shot to be cleared off the line by Saphir Taider.
Inter were reduced to ten men on the hour when Melo picked up a second booking, ruling him out of Saturday’s visit of Roma, but the away side were gifted the breakthrough goal shortly after as Bologna right-back Alex Ferrari misplayed an innocuous-looking through ball, with Ljajic racing onto it before squaring for Icardi to apply a simple finish.
Inter have played well defensively for most of the season, but thy were ripped apart in the dying minutes and it took an incredible reflex save by Inter keeper Samir Handanovic to deny substitute Mattia Destro and send the Nerazzurri back to the summit.