The Premier League side went down to 3-0 to Dortmund away from home at Signal Iduna Park in the first leg of the Europa League last-16 tie on Thursday evening.
"It is difficult to explain but it's a bad day and I'm disappointed with our performance,” Pochettino told BBC Sport. “We did not play well. That is why we lost the game.
Dortmund were the better team and deserved to win, and it is quite honest of Pochettino to admit that Spurs did not play well.
However, the question is why Tottenham did not play well. Was it because Pochettino did not field his best available players?
Kyle Walker, Harry Kane and Mousa Dembele all started the game on the substitutes’ bench, and that decimated Tottenham’s chances of winning the match.
Spurs will hope to bounce back in style this weekend when they take on Aston Villa away from home at Villa Park in the Premier League.
One suspects that Pochettino will field his strongest available team against relegation candidates Villa.