He added that lack of energy, not tactics, that led to defeat against Ajax.
Tottenham have plenty of work to do next week in the second leg in Amsterdam.
He said: “First half we started not in a good way. Ajax showed more energy, it was difficult for us to play. It was our lack of energy,” Pochettino said.
“After we conceded the goal – 25 to 30 minutes in – we started to be in the game. Moussa Sissoko provided good energy.
“Second half we pushed them and tried to create chances. It was an even game in the second half. We are alive. It’s only 1-0 down. We need to believe we can go there and win the game.”
Spurs and Ajax will meet again for the second leg on May 8 at the Johan Cruyff Arena.