Although Esteban Cambiasso gave his side the lead in the first half, a defensive doze gave Catania the win after the Mialn side conceded twice in three minutes in the second half.
Ranieri, speaking to Sky Sport Italia after the game, said: "We needed to pick up points and looked to succeed in that aspect, but in the second half we conceded two counter-attacks straight away".
"We should've stayed compact as we were in the first half, but now we must remain calm and continue working hard, because there's a lot to do," he continued. "I requested that the team keep their shape. Now, we want to react and get back on top."
Ranieri accepted that the penalty awarded to Catania that helped them to the win was reasonable, and praised the referee for his performance.
"The penalty was understandable, as it's difficult for the referee to see the goalkeeper had taken his hands away before Gonzalo Bergessio fell. In my view, he officiated [the match] well."
Inter are now languishing 17th on the Serie A standings after the loss.