Spurs, who played in the Europa Conference League last season, conceded twice late on in a 2-0 defeat by the Portuguese side on Tuesday.
"I think we need to be disappointed because the final result was negative. I watched the game again and I think that we lost a big chance," Conte told reporters ahead of their Premier League clash at home to Leicester City on Saturday.
"It means that we have to do more in the future because we are in the Champions League. We did a very important step (by qualifying) but now we have to show that we deserve to stay there."
When asked if he would leave out struggling forward Son Heung-min against Leicester, Conte said no player was "undroppable".
South Korean Son, a key part of the Spurs attack in recent years, has yet to score in any competition this season.
"No players are undroppable. This has to be clear. If I say this it's only to help my players and to avoid injuries, to try to have players in a good performance. You know you can play one game well, another so-so and another not," added Conte.
He also expressed his delight at defender Eric Dier earning an England recall for the Nations League fixtures against Germany and Italy later this month.
"I'm really pleased because when the national team call my players... it means we are working very well," said Conte. "In this case with Eric, it was a really good achievement for him. I can only tell him that it is to be a starting point."
Spurs are third in the league standings with 14 points from six games. Leicester are rock bottom with a single point.
