With the teams locked at one apiece in the 81st minute Radu somehow hooked the ball towards his own net when trying to pass it out, allowing Nicola Sansone to tap in a goal which hands AC Milan the advantage in the title race.
Champions Inter, who had taken the lead in the third minute with a powerful strike from Ivan Perisic, stay two points behind league leaders Milan with four matches remaining.
Defeat for Simone Inzaghi’s side could be crucial to the race for the Scudetto as Wednesday’s match was Inter’s game in hand on their local rivals.
The evening’s matches were three of the four not played in January following a spike in coronavirus cases across Serie A and Italy in general.
Marko Arnautovic continued his good form by levelling the scores for Bologna in the 28th minute and they went toe-to-toe with Inter until Radu handed the hosts a potentially title-deciding win.
Earlier, Fiorentina’s hopes of a return to European football were hit after they fell to a 4-0 home hammering at the hands of Udinese.
Vincenzo Italiano’s side would have moved into fifth and the division’s Europa League spot had they won.
However first-half goals from Pablo Mari — his first in Serie A — and Gerard Deulofeu set up the 12th-placed away side for a thumping victory which was sealed with stoppage-time goals from Walace and Destiny Udogie.
Fiorentina sit seventh and outside the European places after the heavy defeat, two points behind Roma and level with Lazio who hold the Europa Conference League place, as they seek continental competition for the first time since the 2016-17 season.
They stay seventh thanks to a 4-4 draw between Atalanta, in eighth, and Torino which featured four penalties and an own goal.