Mauricio Pochettino has told the Daily Express that his Tottenham Hotspur squad need to realise they cannot switch off in matches.
Spurs made a brilliant start against Anderlecht in the Europa League on Thursday night, as they raced into a 1-0 lead in Belgium.
However, the Lilywhites took their foot off the gas after their fast start, and they were made to pay for their complacency, with Anderlecht coming back to win the match 2-1.
And Pochettino admitted: "We lost our focus and, after ten minutes, the games changed completely.
"This was our problem tonight and we need to learn. We are very young, but we need to learn a lot.
"When you are the best team on the pitch after 10 minutes and you believe that you can win the game, you need to win the game. Your focus needs to be 200%.
"We need to realise that we need to compete every game, in every single minute, in different games and different competitions, and we need to give our best."
Spurs had controlled the opening stages against Anderlecht, and Christian Eriksen's goal was reward for their dominance.
Erik Lamela and Clinton Njie also missed early chances, and their failure to extend their lead would ultimately prove costly, with the London outfit falling to a disappointing defeat.
Spurs have made no secret of their desire to have a lengthy run in the Europa League this season, but their progress from the group is now in doubt.
Tottenham have taken four points from their first three fixtures in Europe, and they are likely to have to beat Anderlecht at White Hart Lane if they are to make it through to the knockout stages.