Frank De Boer slammed Inter's approach to holding onto a lead after Cagliari came from behind to win 2-1 at San Siro.
Inter's head coach also suggested that striker Mauro Icardi - who was the target of crowd protests about comments he made about the club's fans in his autobiography - would have scored his penalty "on another day" after firing a first-half spot-kick wide, though refused to discuss the striker's situation any further.
The Nerazzuri led through a Joao Mario strike before goals from Federico Melchiorri and a Samir Handanovic own-goal completed the turnaround for Cagliari.
Following the defeat De Boer's men sit 11th in Serie A with just three wins from their opening eight games, a record the Dutchman is not impressed with.
"We stopped playing football once we took the lead," De Boer told the club's official website.
"It should have been the easiest spell of the game for us with greater space to exploit - instead we lost.
"We played well in the first half though we should have made better use of the space in the opposition box. We need to get greater numbers forward from midfield and out wide.
"We didn't have any issues for the first 70 minutes and, although we didn't create many chances, we still hit 20 crosses.
"We didn't play to our potential when we led 1-0 and that's the real reason why we lost the match. We lost too many challenges in the middle of the park and that wasn't down to misfortune.
"I didn't expect this defeat because we didn't see it coming."
Inter vice-president Javier Zanetti warned before the game that Icardi could be stripped of the club captaincy following his comments, but De Boer would not be drawn any further.
"On another day Icardi would have scored from the spot. I don't want to discuss the rest of it," he said.
"We'll assess things tomorrow out of respect for the fans and the player. We must find a solution because the situation is far from ideal."