Barcelona were only 1-0 up when the young Frenchman found the woodwork, but the hosts increased their lead just moments later.
They went on to collect a 4-0 win, enough to overturn the 2-0 first leg deficit.
"When Niang hit the post, I understood everything," the official stated. "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."