The North Londoners lost 1-0 to the Belgian side in the Round of 32 first leg after Jeremy Perbet scored the only goal of the contest at the Ghelamco Arena in the 59th minute.
Spurs will now return to London to prepare for next Thursday's return bout at Wembley which Pochettino, despite being frustrated, feels creates a fantastic opportunity for his players to finally perform in Europe following a run of poor results on the continent this season.
"Very frustrating. I think that the tie is open now, it's not only 90 minutes, it's two games," he told BT Sport.
"First half was even, second half we changed our formation to try and play better. We were better and then we concede a goal but we have some clear chances to score.
"I'm disappointed, yes, because we had two or three clear opportunities before we concede, but the tie is open."
Asked why his side conceded after gaining the ascendency, Pochettino added: "Because it is football.
"Sometimes when you concede some chances the opponent scores, that's the game.
"It's true that maybe it was not a good performance but we need to understand it is difficult to play in the Europa League but we tried to find a way to win the game and go to Wembley in a better position.
"We go next Thursday and fight and try to turn the tie and go to the next round.
"It is fantastic opportunity for us, to make home at Wembley and put pressure on the opponent.
"To demonstrate to us that we can win and play well at Wembley. It will be fantastic to show ourselves that we can do this and be ready for next season."