Tottenham manager Mauricio Pochettino has bemoaned missed opportunities and a loss of focus in their 2-1 defeat to Anderlecht at Stade Constant Vanden Stock on Thursday.
Spurs went ahead as early as the fourth minute through Christian Eriksen, but Guillaume Gillet equalised for the Belgian side soon afterwards before Stefano Okaka struck a 75th-minute winner.
"I'm very disappointed," the Argentine boss said. "With the way we lost focus in the game you always pay because it changes the dynamic of the match.
"I think that we started the game very well, scored for 1-0 and then created some clear chances to score two or three goals, but you have to look at how the game changed on set pieces.
"On the first chance they scored, after that there was another big chance for them and it was difficult to come back into the game and play like we started the match.
"We need to learn about that. We are very upset about the result and very disappointed but we need to take the positive things, we need to learn to kill the games off and play at 200 per cent every game. When you lose your focus you will pay on the pitch."
The result leaves Spurs level with Anderlecht on four points in group J, one point behind Monaco. Their next match is back in the Premier League against Bournemouth at Vitality Stadium on Sunday.