Lucas Tousart struck the winning goal during the few minutes when Matthijs de Ligt was off the field for treatment to a head wound.
“We got the first half wrong. We were second to every ball. In some games, you pay for the tiniest error. We were too stretched out, not aggressive enough and that is an issue of mentality, we should've woken up earlier," Bonucci told Sky Sport Italia.
The captain seemed to realise the atmosphere was wrong before kick-off, as he was spotted shouting at Blaise Matuidi during the warm-up.
“I didn't tell Matuidi off, I told him the team includes those who are on the bench and we must all be ready to contribute. I could tell something wasn't switched on with the XI starters either. We've got to sort this out, as these are the things that make the difference."
He added “They hit us when we were down to 10 men, that was the little moment when they were smart to hurt us. Now we have to get our heads focused on what awaits us on Sunday, then we absolutely must qualify in the second leg."