Zola was Chelsea's inspiration in their 4-1 triumph in the fourth-round tie, setting up first-half goals for Jimmy Floyd Hasselbaink and Emmanuel Petit.
Chelsea's reward for their impressive victory is a visit later this month to Old Trafford, where they won 3-0 last season in the Barclaycard Premiership, and Zola warned that his side have not yet reached their peak, despite having taken their unbeaten run to nine games.
"It was a difficult cup tie. But we coped very well - and I think everyone is growing up, me included," he said. "That is good news because things can only get better. When you have such a confidence everyone can do their best.
"We have had really good times in the past here. Especially when I first came here we had a fantastic atmosphere, but now it's getting better and better.
"When you put together so many results in a row there is something behind it. The team hasn't got to its best yet, so that is very good news for us because we are building something.
On paper, it's one of the best teams I've played in at Chelsea. But that doesn't mean anything. What really counts is how you perform on the pitch."