Juve won the first-leg 3-0 at Parkhead.
"If you didn't watch the game then you might think we cruised to victory," the Italian international commented.
"We have to be honest though as the game hinged on a couple of decisive moments. But it's a positive sign that things went our way and also that we took our chances."
"We set out to attack them, but struggled to get into our rhythm so we had to settle for what we could manage, which was to defend well and break forward on the counter.
"We did a great job of it too - we were clinical and ruthless.
"Are we already through? It's not good for us to be too relaxed. We must remain focused and keep showing Celtic the respect they deserve."