Andrea Stramaccioni bemoaned the "severe error" which led to his Inter Milan side conceding the only goal of the game as they lost 1-0 to Parma at the Stadio Tardini.
The coach has now seen his team fail to win any of the three games since they ended the 49-match unbeaten run of Juventus earlier this month, taking just one point during that run.
Stramaccioni said that the Parma defeat was entirely down to his team's mistakes.
"In the second half we did not do very well, [Nicola] Sansone's goal started from one of our own throw-ins and then we just let him go," he told Sky Italia.
"It was a severe error. It was obvious that we lacked something in the middle of the pitch. We should have avoided that goal."
One player who did earn praise from Stramaccioni was full-back Yuto Nagatomo, whose form for Inter and for Japan has earned him a nomination for the Asian Footballer of the Year award.
"He's one of the players who has improved the most since I've been here and he can still become even better, he's humble and he works hard," Stramaccioni said. "We're holding on tight to him.
"The individual work that he's doing with [Inter assistant coach] Beppe Baresi is also important. If he also becomes good with his left foot then he'll be even more dangerous."