Mauricio Pochettino said he is searching for answers to Paris Saint-Germain's post-Champions League struggles after the Ligue 1 holders were stunned by lowly Nantes on Sunday.
PSG missed the chance to go top of the Ligue 1 table following a shock 2-1 home defeat against relegation-threatened Nantes.
Pochettino's side looked on track to claim maximum points thanks to Julian Draxler's 42nd-minute opener, but two unanswered goals in the second half lifted Nantes to their first win away to PSG in Ligue 1 since 2002.
PSG have now lost their last three Ligue 1 fixtures immediately after Champions League clashes – the French giants fell to Lyon 1-0 following their 5-1 rout of Istanbul Basaksehir on December 9 and suffered a 2-0 defeat against Monaco after crushing Barcelona 4-1 at Camp Nou on February 16.
Now, after being shocked in Paris on the back of their 1-1 draw with Barca in the second leg of the last-16 tie on Wednesday, head coach Pochettino told reporters: "Surely there must be some connection, which we have to find.
"After the match in Barcelona, we played with practically the same players and you could think that we lacked energy. But today we brought in new players.
"It's clear that the energetic and stressful demands of the Champions League are hard for the team, especially when the matches are as physical as tonight's.
"In the first half we absolutely dominated the match. In the second half, when we conceded the goal, it cost us a lot and that's when it affects us to have played a match as important as the one we played here on Wednesday."
It was a forgettable outing for Pochettino and last season's Champions League finalists PSG, who are second and three points behind leaders Lille through 29 Ligue 1 rounds.
PSG have lost two consecutive home games in Ligue 1 for the first time since November 2012.
Pochettino is the first PSG coach to lose three of his first 12 Ligue 1 games since Antoine Kombouare in 2009 (four).
Meanwhile, Angel Di Maria and an as of yet unnamed PSG team-mate had their houses broken into during the Nantes showdown.
Di Maria was taken off midway through the second half at Parc des Princes, after a curious discussion between officials – including sporting director Leonardo, who appeared visibly angry – in the stands, and the coaching staff on the touchline over the phone.
The substitution caused confusion - not only off the pitch, but also on it. PSG conceded just before Di Maria went off, and then again nine minutes later to throw away their lead and ultimately lose.
On the Di Maria case, Pochettino added: "There are situations outside football that we should be aware of.
"The footballers are the first to know the disappointment of losing the three points but there are other situations, beyond football, that the footballers and we should be concerned about, I think you already know that."