Massimiliano Allegri drew his conclusions after his side were beaten by Sassuolo's injury-time goal.
"We could have taken the lead in the first half but then two minutes from the end Sassuolo scored. We weren't solid enough defensively. After equalising we lost our shape and allowed them a couple of chances – one of which they took advantage of right at the death.
"We certainly need to be scoring more goals because one per game is not enough. We need to get the balance right between attacking and defending at all times. We must think long and hard about this defeat because we can never afford to drop our guard. We clearly have to do much better than what we're doing at the moment. Mentally we must be much stronger as a team.
"Now it's crucial we put this result behind us immediately because we have another very tough game on Saturday."