London Irish qualified for the quarter-finals of the European Challenge Cup as top seeds following their 23-9 Pool One win at Rovigo on Saturday.
Tries from Kieran Roche, Adam Thompstone and Dominic Shabbo held off a stubborn effort from the Italians.
Roche set the second-string Exiles on their way before half-time at the Stadio Mario Battaglini.
But the visitors were reduced to 14 men when lock Gary Johnson was sin-binned and three penalties from German Bustos kicked Rovigo back into the game.
Toby Booth's men went back in front when Shane Geraghty added a 57th-minute penalty before Thompstone gave the Guinness Premiership side breathing space.
It was only the 21-year-old academy wing Thompstone's second try at senior level and the Exiles maintained their unbeaten run when Shabbo crossed three minutes from time to confirm their status as top seeds.