The Nerazzurri slipped 12 points behind leaders Juventus and down to fourth spot following the loss to Siena, while the race for the top three and Champions League football has heated up, with AC Milan closing the gap along with Fiorentina.
“It is without doubt an ugly defeat and we are very angry,” Stramaccioni told Sky Italia.
“We had a few players missing but there are no alibis for a poor performance. Despite the fact we lost, we are still only three points away from the Champions League places, so we cannot give up and have to keep fighting.
“We can still achieve our objective but must find ourselves again, as we are still the team who in November were a point off the leaders and had great ambition.”
Stramaccioni added: “We are going through a difficult moment, but I am sure we will get out of it.
“It was a bad performance and we deserved the defeat. In these cases either you cry and despair or you try to get back on track, I prefer the latter.”