Tottenham secured their Premier League status on the final day of the season after a hard-fought campaign that saw them win three of their final seven matches, alongside two defeats, under De Zerbi's leadership. The crucial late surge proved just enough to keep them above West Ham.
De Zerbi replaced Igor Tudor signing a long-term deal and immediately making his presence felt at the club.
"Since he's come in, he's been brilliant," Maddison told Sky Sports at the Maddison Invitational charity golf day.
"Without him coming in, there is a serious possibility that we would have gone down.
"He switched the mindset, he switched the passion. The passion he shows every single day rubbed off on the players and I think you could see that in the last few performances.
"Yes, maybe we could still be better in terms of quality in some of the moments we were producing. But the passion and the intensity just switched."
Maddison was keen to stress that effort was never the issue within the squad, but that De Zerbi's arrival helped unlock a higher level of performance at a crucial stage of the season.
"I never questioned the group's mentality. I didn't see a group who weren't willing to try, it just needed Roberto to come in and fine-tune it and get it out of some of the players. And he managed to do that, so big credit to him," he added.
Meanwhile, Joao Palhinha has also underlined his importance in Tottenham's survival push, scoring decisive goals in key matches, including the final-day win over Everton.
Palhinha, currently on loan from Bayern Munich, expressed a strong desire to remain at the club permanently, saying he already feels at home in North London.
"I always shared my thoughts. Since the first day I arrived, I feel at home. From the supporters, the crowd. Top club," he said.
"Who doesn't want to play for Tottenham and stay here? I have everything here, but this is like a marriage."
De Zerbi has also made it clear he would "100 per cent" like to keep Palhinha beyond the season, with Tottenham holding an option to make the move permanent for around 30 million euros.
Palhinha added that the difficult season could still prove valuable for the club's future direction, urging reflection and improvement ahead of the next campaign.
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