Mauricio Pochettino insists he does not regret fielding a weakened team despite Borussia Dortmund thrashing Tottenham 3-0 in the Europa League last 16.
Harry Kane was left on the bench at the Westfalenstadion as Pochettino played Nacer Chadli up front and Ryan Mason and Tom Carroll in midfield and handed 18-year-old Josh Onomah only his fourth start of the season.
The decision to rest some of his top players, with Mousa Dembele also on the bench and Eric Dier left at home, backfired as Marco Reus' double and Pierre-Emerick Aubameyang's header all but ended Spurs' hopes of progress.
"No regrets," Pochettino said. "I'm very disappointed with the result but we didn't have a very good day. It was a bad day in the office.
"When we analysed the squad and picked the starting eleven we tried to win the game. It started okay but it was a really bad day for us."
The defeat leaves Tottenham with a mountain to climb in the second leg at White Hart Lane but also means Pochettino's men are now winless in three matches, following last week's loss at West Ham and draw with Arsenal.
A trip to bottom-of-the-table Aston Villa on Sunday offers the perfect chance to bounce back as Spurs look to resume a title challenge that has them sitting five points behind league leaders Leicester.
"I am not worried, I know the circumstances," Pochettino said.
"We need to accept some periods during 10 months that we will have good and bad times. We have always had things good.
"It has only been in the last few games not so much. The players are very sad but we have to take the positives.
"We have a big challenge against Aston Villa on Sunday. We are in a very good position in the league and we will do our best."
Placed second in the table, Tottenham will hope to face similar opposition more regularly in the Champions League next season.
But while similarities were drawn between the two sides' style of play, league position and low-budget squads before kick-off, Dortmund demonstrated Spurs still have a way to go to compete with the European elite.
"I think they had a good experience but we spoke yesterday and I told them that to play at the next level you compete with teams like Borussia Dortmund, which is a real test," Pochettino said.
"It was good for us to feel what a game in the Champions League feels like.
"It was disappointing because the result was bad but to build a group for the next few years to compete at the level of Borussia Dortmund is very difficult.
"We were disappointed but it was a very good test for us to compare the level."
Dortmund coach Thomas Tuchel admitted his side "dominated the match" but was not surprised by Pochettino's team selection.
"We were expecting it a bit because they did it before," Tuchel said.
"They are close to Leicester in the league. They have a chance to win an extraordinary title so they made some changes.
"But we made some changes and we didn't lose our performance. We did not care so much."
Ilkay Gundogan, whose agent was pictured this week meeting Manchester City sporting director Txiki Begiristain, did not play, with Dortmund insisting the midfielder had a problem with his foot.
Another reported target for England's top clubs is Aubameyang, who scored his 33rd goal of the season against Tottenham, but Tuchel is confident the striker will stay.
"Everybody has to accept he has a long-term contract without the possibility to step out," Tuchel said.
"I can imagine a lot of clubs see his performance, his talent and his personality and imagine Auba plays for them.
"But we have our own goals. We want to keep this squad together and develop into something extraordinary. We want to have players that lift our status.
"It is not our goal to give him away. I cannot see the future but if you ask me as a coach, I'm very happy he has a long-term contract."
On Gundogan, Tuchel added: "We don't know. We looked at his foot and he could not play. Maybe if we gave him a special shoe that would allow him to play we could try that."