Manager Mauricio Pochettino rejected accusations that Tottenham are mentally weak following his team's 2-1 Europa League win at Asteras Tripolis.
Leading 2-0 at half-time, Spurs faded after the break, just as they did in a 2-1 Premier League defeat by Newcastle.
The loss to the Magpies promoted England coach Gary Neville to describe Spurs as "weak".
"If you have a weak mentality, it would be there in 90 minutes, not only in some moments," said Pochettino.
Tottenham were apparently in control against their Greek opponents after goals from Andros Townsend and Harry Kane.
But they were left hanging on for victory after defender Federico Fazio was sent off in conceding a late penalty that was converted by Jeronimo Barrales.
"You need to show you are ready for 95, 96, 97 minutes in the game - it's true, it's true," added Pochettino.
"We need to work very hard with our mentality to be focused on the game."
Spurs are level on points with Besiktas at the top of Group C and progress to the last 32 if they beat Partizan Belgrade at White Hart Lane on 27 November.