Mauricio Pochettino admitted he got his tactics wrong in Tottenham's 1-0 defeat to Ajax in the first leg of their Champions League semi-final.
Slicker and sharper than their hosts, Ajax's crop of talented youngsters pinned Spurs back during an utterly dominant opening 30 minutes where Pochettino had set his team up in a 5-3-2 formation.
Ajax were able to find space in midfield and got their deserved goal when Donny van de Beek netted a priceless away goal.
"Watching now of course I can accept it was a mistake the shape we used," Pochettino told BT Sport.
"But there were not too many options. I am not happy - you cannot guess what happens if we play in a different way. But our lack of energy and [we were] a little sloppy.
"It was the not the shape that conceded the goal. Our approach to the game was not good. I am the manager so I have responsibility."
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- Vertonghen forced off after head clash
A head injury to Jan Vertonghen forced the Spurs boss to change things before the break, allowing for the introduction of Moussa Sissoko to beef up the midfield and switch to a 4-4-2.
Spurs looked a different side, finding more opportunities to force Ajax back, although they failed to produce the quality in the final third to score an equaliser.
"After we conceded the goal - 25 to 30 minutes in - we started to be in the game," said Pochettino.
"When Moussa Sissoko was in, he provided good energy and we played better.
"Second half we pushed them deeper and pressed in a way we wanted to press. The second half gives us hope for the second leg."
'Poch will learn from this'
Former Premier League defender Liam Rosenior speaking on The Debate: "What would Mauricio Pochettino have learned? Not to play 3-5-2 against Ajax! As soon as they changed to a diamond, they pressed Ajax higher up the pitch.
"Being 1-0 down is difficult - Ajax have some incredible young players - but Pochettino will have learned some tactical lessons. And having Heung-Min Son back will be huge because Ajax leave big spaces in behind."