Both players, speaking after Sunday's 1-0 home win over Everton secured Premier League survival, described a squad stripped of belief and on a trajectory that pointed only downward.
"Without that appointment, disaster could've maybe struck," Maddison said. "It didn't, and he takes a lot of credit for that because of the work he's done behind the scenes, on the training pitch."
De Zerbi took over in late March as Spurs' third manager of the season, inheriting a side that had gone 14 league games without a win — nine under Thomas Frank and five under interim boss Igor Tudor — and had slipped into the relegation zone.
Maddison, who missed the majority of the campaign with a serious knee injury, returned for substitute appearances during the run-in and said watching from the outside gave him a clear view of what the manager changed.
"I stay at the training ground the night before every home game to get a good night's sleep and he's there at nine pm with all his staff, six of them and the tactics board," he said. "It's nine pm and we've already had four or five meetings on each game. He's just obsessed with football."
Gallagher, signed for £34.7 million from Atletico Madrid in January, had a difficult start to life in north London.
He made his debut in a home defeat to West Ham, then found himself moved from central midfield to the wing by Tudor before being dropped to the bench entirely.
"It was such bad times," he said. "We were losing most of our games. I wasn't playing. When I did play, I didn't do very well. I was playing out of position."
De Zerbi addressed that immediately. He held individual meetings with players and showed them YouTube highlight reels to rebuild their confidence, with Gallagher one of those targeted.
"One of them was titled 'Bossing the midfield — Conor Gallagher' or something like that," he said. "It's funny because I'd seen that before and he's just there showing me. He did that with a few of the lads. I found it funny, but it was one of his many ways to get players' confidence back and it helped me."
Spurs won three and drew two of their final six games under De Zerbi to finish on 41 points, two ahead of relegated West Ham.
Gallagher was emphatic about the transformation.
"From the first day or two he had everyone under his wing. Everyone trusted him instantly and everything he was doing, it was like, 'thank God he's come in'. Straight away. I can't speak highly enough of him for what he's done for the club, for the team and for me individually."
Maddison added that the manager's authenticity was what landed with the squad.
"When you see the man who's steering the ship, when he's genuine and not just saying it for the sake of a motivational meeting, you can tell he means it."
De Zerbi has signed a five-year contract as Tottenham head coach. The rebuild starts now.