Mauricio Pochettino acknowledged he made a "mistake" with his original set-up while adding it was a lack of energy, not tactics, that led to defeat against Ajax.
Tottenham fell 1-0 to the Dutch side on Tuesday, leaving plenty of work to do next week in the second leg in Amsterdam.
In Tuesday's first leg in north London, Pochettino opted to start with three central defenders, with
The decision came as a result of Spurs' injury and suspension issues, with regulars Harry Kane and Son Heung-min headlining the absentees.
Ajax scored early through Donny van de Beek, who waded his way
And while the decision to play three-at-the-back was one Pochettino regretted, he says his side also needed to show more fight early on to prevent conceding that valuable away goal.
"Watching now of course I can accept it was a mistake the shape we used. But there were not too many options," said Pochettino told BT Sport.
"I am not happy - you cannot guess what happens if we play in a different way. But our lack of energy... and a little sloppy.
The plan shifted when Vertonghen was forced off with a concussion after
Vertonghen attempted to play on, but it was clear he was dealing with concussion symptoms, prompting the introduction of Moussa Sissokho.
And the France international's presence helped steady the ship, though it was not enough for Spurs.
"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.