Mauricio Pochettino admits Tottenham have run into a "bad period" after they suffered a surprise defeat to Gent in the Europa League last 32.
Spurs' disappointing night in Belgium was also compounded by Harry Kane sustaining a knock to his knee.
Kane later suggested the injury was not serious but the striker is still unlikely to be risked against Fulham in the FA Cup on Sunday.
Jeremy Perbet's second-half winner secured Gent a 1-0 victory at the Ghelamco Arena and leaves Tottenham with work to do in next week's second leg at Wembley if they want to progress.
The loss also continues a worrying dip in form for Pochettino's men, who have won only two of their last six matches after winning their previous seven in a row. They have scored only once in their last four.
"That is the reality. It sometimes happens," Pochettino said.
"Good and bad things happen. We've started now a bad period. I hope, on Sunday, we break that negative run."
Pochettino added: "It's not about confidence. Not about confidence. We were wrong with our judgement if we think, because of Liverpool, it's only about confidence."
Tottenham's poor performance was all the more surprising given Pochettino fielded a strong team, that included the likes of Kane, Dele Alli, Mousa Dembele and Toby Alderweireld.
He is likely to rest at least some of those against Fulham, including Kane, who took a knock to his knee during a collision late on.
Kane was seen walking out of the stadium, seemingly unhindered, and when asked whether he would be fit for the game at Craven Cottage, he said: "Yes, it should be fine."
Pochettino, however, suggested he would rotate for the fifth round tie.
"I think we need to assess some players. Harry Kane got a knock on his knee, different players too," Pochettino said.
"We need to refresh the team and we'll do that. Tottenham will play in Fulham on Sunday. It's not the name of the players, it's the club.
"We'll see, but we'll put out a team with aims to win and be ready again to compete."
Kane was one of a number of Tottenham players to endure an off night against Gent, who sit a lowly eighth in the Belgian top tier, but fully deserved their victory.
It was a special day too for their manager Hein Vanhaezebrouck, who was celebrating his 53rd birthday on Thursday.
"We're happy," Vanhaezebrouck said.
"We believed in our own qualities. We were not afraid to play, and that is a victory in itself for me. I told them to believe it was possible to win. To do that was the cherry on the cake for me."
Tottenham will still be favourites to turn the tie around in front of their own fans at Wembley but the national stadium has not proven a happy hunting ground for the team this season.
They lost two of their three games there in the Champions League group stage, a record that proved key to them failing to go through.
"Going to Wembley is mythical," Vanhaezebrouck added.
"The most important thing for us was to make it a game, to avoid losing 4-0. That can happen for the best.
"We saw it this week. But now we can really make a game of it. We made the return game a very interesting game next week."