With Barcelona leading 1-0, M’Baye Niang darted clear only to see his shot come back off a post with goalkeeper Victor Valdes beaten.
Lionel Messi rattled his second of the night into the Milan net seconds later before the hosts romped to a 4-0 win rendering Milan's 2-0 success in the first leg at the San Siro ultimately obsolete.
“When Niang hit the post, I understood everything,” said Galliani. “On nights like this, certain signals are crystal clear.
“I immediately said to Barbara Berlusconi after the Niang chance that we risked going 2-0 down… football is like that.”
Galliani insists his side must recover from the mauling at Barca when they return to domestic matters against Palermo this weekend.
“We now have to think about the Milan-Palermo game,” he added. “We can’t have a repeat of what happened last season after the game in Spain when we lost at home to Fiorentina.”