He wasn't. Juve had Boksic who was a monster, and also a young Vieri who was very good. Boksic and Vieri were the preferred starters. For example, Del Piero came on a second half substitute in the CL final vs. Dortmund. In 98, Vieri was sold for a record fee to Atletico and Inzaghi was brought on, and Del Piero was progressively taking on more duties. However, Zidane was still the regista. He made the team click.
As for Juve's collapse in 98, it due to many factors - Lippi had moved on and Ancelotti was cutting his teeth and generally had no clue. He didn't know what to do with Henry for example (sold him to Arsenal, after playing him as a winger) and surrendered two subsequent scudettos whilst being in the lead. He was fired afterwards.
As for Nevded, he was good, but Zidane was miles better. Juve sold Inzaghi / Zidane and bought Nedved, Thuram and Buffon. In terms of style of play it was Capello 101 - a combative, rigid, and much uglier team. I used to follow Juve, and couldn't warm up to that team.