Samp came from behind with Giampaolo Pazzini's brace to win 2-1 at the Olimpico.
"If we had drawn this match then it would've been the same thing. We were in charge of our own destiny and now we aren't," confessed Ranieri. "We must carry on as if nothing has happened. We had probably our best first half performance since I've been here.
"Unfortunately we did not remain as solid after the break and Samp took advantage. I tried to create a few more crosses for Luca Toni, but we couldn't find the goal."
On the title race, Ranieri added: "I don't know where Inter are likely to drop points. All I know is that we must give our all. The sad fact is our fate is no longer in our hands.
"The Scudetto will be decided in the final round. We have fallen and must get back on our feet. Luck was on our side in the derby, but not tonight.
"We certainly need to end the season well. Inter are strong, so compliments go to them and to Roma for an impressive campaign."