Frank de Boer was frustrated as his Inter players "lost their heads" in defeat to Hapoel Be'er Sheva, though he was happy with his side's first-half performance.
Having beaten Pescara in Serie A on Sunday, Inter were looking to get their Europa League campaign off to a winning start.
However, they were outplayed by Be'er Sheva who ran out comfortable victors, leaving De Boer rueing his team's mental weakness.
"Before their goal, we had control," he told Sky Italia. "Then they won a free-kick when [Jeison] Murillo was off the field, three of them were good at blocking our players and they scored.
"As happened in Pescara, we lost our heads a little and granted them several opportunities.
"We started well and I was convinced we could score. I do not understand how anyone can lose their head after conceding a goal. We must know that there has to be organisation and I do not understand why there was a change."
Inter's difficult start to the season looks set to get tougher this weekend, with Juventus the next visitors to San Siro.