The Nerazzurri remain without a victory in 2014 after losing at the Luigi Ferraris, and they are now 11 points adrift of the Champions League places.
Luca Antonelli headed home a Rossoblu corner with just seven minutes to go at a saturated Luigi Ferraris to consign the Nerazzurri to a third defeat in four games in all competitions.
Mazzarri was utterly bewildered by this latest loss, which left his side 11 points adrift of third-placed Napoli in Serie A.
"I cannot say anything to the boys," the Inter coach told Sky Sport Italia. "We could only have done more in front of goal and have been more attentive on their goal. The pitch was difficult and we faced a strong and organized opponent. We had a good game but it was not enough.
"Unfortunately, it is a moment in which everything is going wrong. We are throwing away points. Had we been at 37 [points instead of 32, following last week's draw at home to Siena], everything would be different, but now we just have to keep working."