After finishing third last season and scoring 98 goals in the process, Atalanta have struggled for consistency and it was widely reported last week that coach Gian Piero Gasperini had fallen out with team captain Alejandro Gomez.
"We played the first half like warriors, and the second half like kids with a completely different attitude," said furious Roma coach Paulo Fonseca.
"If we had maintained the attitude we showed in the first half, we could have beaten any opponent. The team let their heads drop for no reason, it's hard to understand what happened today."
But the game changed completely after Josip Ilicic came on at half-time for the hosts.
Two minutes later, Luis Muriel, who had just come on as a substitute, intercepted a Jordan Veretout pass to add the third and Ilicic scored the fourth himself with a solo goal five minutes from time, weaving his way through the Roma defence.