Spurs lost 2-1 to lowly Newcastle at home on Sunday, leaving the Londoners in 11th place on the Premier League table with 11 points from nine games - five points behind fourth-placed West Ham.
Pochettino's men took the lead through Emmanuel Adebayor in the 18th minute but Shola Ameobi equalised before Ayoze Perez grabbed a 58th minute winner at White Hart Lane.
"We need to improve a lot [to get in top four]. It's not tactical or physical it's our mentality," said Pochettino.
"It's a big hit for us. We need to be more strong and to have more focus. Today in the second half, straight from the kick-off we conceded a goal. It's crazy.
"It was a shock because after that it all changed in this moment. Seven or six seconds before, all was happiness and there was good energy in the stadium. Our supporters were happy and believed that we would get a good result.
"But in this moment, I think it was difficult to manage in our head. Our heads had gone and we started to take rash decisions on the pitch. It was difficult because 12 or 14 minutes later, we conceded again another goal. At 2-1, it was difficult."