Eder accepts Inter suffered an "ugly" Europa League home defeat to Hapoel Be'er Sheva that has knocked their confidence.
Frank de Boer's men won in Serie A at the third time of asking against Pescara last weekend, but they were deserved 2-0 losers as the Israeli champions bossed Thursday's game at San Siro.
Italy forward Eder spoke of his frustration on another testing evening for De Boer in Milan, adding that his team must now move on from this disappointment.
"In football there are no easy games anymore," Eder told Sky Italia after the match.
"In the first half we made opportunities and unfortunately we could not take the lead. In the second half, after their goal, we also fell on a mental level and did not have a reaction. It was an ugly defeat.
"It is a shame because a victory here after the one in Pescara would have given us so much confidence, but we have to work."
Inter's next test is a vital league clash with Serie A champions Juventus on Sunday as they seek a first home win of the season.