Antonio Conte did not believe Tottenham deserved to lose to Sporting CP but suggested their late collapse was a reminder of the limited room for error in the Champions League.
Tottenham were below par in their Group D encounter but looked set to settle for a share of the spoils at Estadio Jose Alvalade.
However, Paulinho's 90th-minute header from a near-post corner and a fine individual goal from Arthur Gomes gave Sporting a win that puts them in control of the group on six points.
Spurs now have plenty of work to do in a pair of games with Eintracht Frankfurt after falling to a result Conte did not view as reflective of their performance.
"I think the game in the first half was in balance between the two teams," Conte told a media conference.
"Honestly, in the second half, we felt the opportunity to get the three points and win the game. We had the chances to score, but in the end we are talking about a loss.
"For sure, we can do much better in those last few minutes. We conceded a counter-attack with a one-versus-one. It was the player who shot and Hugo [Lloris] made a really good save, maybe the most difficult save of the game.
"After this, the corner we conceded a goal. I always say to my players that the details move the final result. I think that maybe we didn't deserve to win, but at the same time we didn't deserve to lose this game tonight.
"We try to analyse the game very well with the players to understand what were the positive things that we did and what are the negative situations that we can improve for the future.
"For sure, this level is a high level, the Champions League. Against Marseille was another tough game. Tonight was another tough game. We need to know that if we want to have the ambition to go to the next round, we have to fight, we have to fight a lot, especially in this group, because in this group I see a lot of balance between the teams.
"The Champions League level is high. You know that you have to reduce the mistakes because in the end you can pay. Tonight we paid a lot.
"Now we are talking about a loss, a loss against a team that showed they are used to playing this competition. It's a pity, and we have to analyse the game well, and all the players they know very well that this level is an important level.
"It's right to continue to improve this process. Don't forget that last season we lost against Mura in the Conference League, and I think we made an important step forward."