Third-placed Spurs are with out a win in the last four league games.
“It is difficult to comprehend why we shifted so much from the first half on the moment,” said Pochettino.
Discussing BBC Match of the Day, he also added:”We were better at everything but we lost the match. We didn’t take the next half seriously.”
Pochettino’s side led at the rest during Harry Kane’s 200th career goal but Yan Valery and James WardProwse scored in the space of 5 minutes to finish the turn around.
“I’m quite disappointed but this is the reality,” Pochettino added.
“Folks talk about perceptions however, we will need to realise if you do not fight with intensity for the Champions League, you could fight.
“the fact is you can’t just play 45 minutes – we’re Champions League group, even in the quarter finals. The team does not need the experience of competing in such conditions, so it is excellent to know but we need to fight each and every opponent.
“We are fighting against big chunks [for top four]. We had a big gap – welcome into the reality – and that we are under great pressure to win games. It’s a massive challenge for us”
Tottenham haven’t won a match in the Premier League since they beat Leicester not exactly a month ago. But, they did advance to the Champions League quarter-finals by beating Borussia Dortmund home and off.