The Nerazzurri fancied their chances of climbing back into third place with an easy victory over lowly Siena, but Giuseppe Iachini's men had other ideas.
Antonio Cassano may have responded quickly to Innocent Emeghara's opener, but Inter were all at sea when Alessio Sestu and Alessandro Rosina put Siena out of sight, the latter scoring from the penalty spot as a result of Cristian Chivu's dismissal.
Inter have taken just one win from seven games but Stramaccioni is encouraged by a race for Europe that remains wide open.
Stramaccioni told inter.it: "I've never liked making excuses and I can never find the right ones anyway. I was an ugly defeat and that's all there is to say.
"But thanks to this strange season we are still in the mix. I for one will be putting a brave face on it.
"The blame lies with everyone but I represent them, so I will front up, but I am sure we can still achieve our objectives all the same."
A run of seven straight victories through the autumn had established Inter as one of Juventus' closest challengers, but those lofty aspirations now seem a distant memory.
"We have to rediscover the Inter side that had drawn to within a point of the leaders," Stramaccioni added. "We are the same side, I'm the same coach.
"The fact that the team did not give up when they went down to 10 men was really important and the lads used all the energy they had.
"We must go back to the way we were and reverse this negative trend. It's an ugly and bitter one but I am convinced we can change."
Siena remain reluctant to talk about surviving in the top flight despite closing to within four points of 17th place.
Club director Massimo Mezzaroma told acsiena.it: "It's an important result against a great team. There have been a lot of good performances lately but we didn't earn any points from them. I'm really happy with this performance and for the lads, they deserve this win."