Spurs are bottom of the table after losing 1-0 at Brentford and 2-0 at home to Newcastle without scoring, despite investing heavily in their squad during the transfer window.
The latest defeat also exposed Tottenham's long-standing difficulty responding after falling behind. Anthony Elanga opened the scoring in the second half at the Tottenham Hotspur Stadium before Yoane Wissa added a second, with De Zerbi admitting his team lost confidence after conceding.
"After the goal we conceded, I think we lost the confidence. The reaction was not good enough," he said.
Tottenham have now failed to win a Premier League match after conceding first since November 2024, when they came from behind to beat Manchester City 4-1 under Ange Postecoglou.
De Zerbi believes addressing that mentality is more important than any further recruitment before Tuesday's transfer deadline.
"You are right, yes. It's the biggest problem, it's the biggest challenge, the biggest target is to find the right mentality," he said.
"Unfortunately you can't buy the mentality in the transfer market."
One player at the centre of the rebuild is Sandro Tonali, Tottenham's 100 million pounds signing from Newcastle. The midfielder endured a difficult return to St James' Park, with the home supporters booing his every touch before he was substituted in the 75th minute.
De Zerbi defended the Italy international, suggesting his physical condition remains an issue.
"Sandro Tonali is another crucial player. He is not in the right condition, in the best condition yet, maybe physically," he said.
The Tottenham boss also confirmed that Richarlison, Kevin Danso and Pape Matar Sarr have asked to leave the club. Sarr is recovering from a minor hamstring problem, while De Zerbi said the other two are available to pursue moves before the deadline.
"I love these players, and they are great guys," he said. "But if they want to leave, they have to bring the economic solution to the club, and they can go and we find other players."
Despite the poor start, De Zerbi insists there is no panic and remains convinced the squad can develop into a successful team.
"I can't change my opinion for two defeats," he said. "I have big, big confidence in the qualities of my players."
With Tottenham having finished 17th in each of the past two seasons, the pressure is already building, but De Zerbi believes the current project is fundamentally different.
"The plan, the project of the club, what we are doing to rebuild a new team with new players, is a great plan, is a great project, and I have no doubt about the result of this project."
