"I am so happy for him," the West Ham manager told Radio 101. "Mind you, he didn't even need that performance against Real Madrid to be considered one of the greatest in the world in his position."
Del Piero turns 34 on Sunday and won back-to-back Serie B and A Capocannoniere titles before the return to the Champions League.
Zola added: "Del Piero has always shown an immense strength of character every time he hit difficulties. For the past two years he has been playing at an extraordinary level.
"I am not surprised, because it is normal that after hitting 30 there is a period of adaptation to the new physical state."