It means PSG remain winless in their four games against the other sides currently in Ligue 1's top-four positions this season, losing 3-2 and 2-0 to Monaco, 1-0 at home to Lyon and drawing 0-0 at Lille.
Verratti, outstanding against Barca, came on as a 55th-minute substitute in Paris, going on to attempt 43 passes – with an accuracy of 81.4 per cent – and create one chance.
"It was a difficult match; they have very good players, we knew it. It's the kind of match you have to win," Verratti told Canal+. "We are starting to lose a lot of opportunities for the title.
"We concede a goal after five minutes, that can't happen. We have to understand that there are great teams in France.
"We must always play as a team; no one gives us gifts, the other teams are playing the game of their lives against us.
"You have to rise to the occasion, deserve it, play 100 per cent, because otherwise you won't win."
After scoring a hat-trick against Barca, Kylian Mbappe turned in a disappointing performance against his former club.
Since he joined PSG in 2017, Mbappe has scored seven goals against Monaco in Ligue 1; he has only found the net more often against Dijon (nine) in the top-flight during that period.
However, he failed to shine on Sunday. The 22-year-old did not register an attempt on goal and created just one chance.
Pochettino refused to pin the defeat on Mbappe, though.
"In football there are many situations, different opponents, different circumstances," Pochettino told a news conference.
"Football is not mathematical. We weren't at the level today to win at the collective level, not just Kylian. If we have to find a responsible, it's me."