"Qualification is obtained over 180 minutes and so far we've only played 90," he told reporters on Tuesday. "There are another 90 to go against a strong, proud team who will want to make amends for first leg.
"The first leg has given us a seriously good chance of reaching the next round, but in football, there are always surprises. Celtic are a side capable of creating plenty of danger, we need to be fully focused. It's an important test.
"They are a great team and [Neil] Lennon is doing a superb job. Everybody at Juventus should be proud of what we're doing. We need to keep it going. Not succumbing to the pressure of Celtic Park was an excellent test of the team's maturity and gives me great confidence. It's not easy to play in hostile environments."
Juventus are currently six points clear at the top of Serie A.