Mancini’s side were top of Serie A heading into the winter break, but have won just two of their seven league matches since the turn of the year.
Inter now sit fifth in the table, some 11 points behind leaders Napoli but only two adrift of Roma in the third Champions League spot.
The Milan club travel to play fourth-placed Fiorentina – who sit just one point above them – on Sunday and Mancini believes a win in Florence could reignite their season.
“This won’t be a decisive match, but it will be important and this is a decisive stage of the season,” Mancini told his pre-match media conference.
“Over the last few weeks we have thrown away all the extraordinary work we’d done in the previous five months.
“We started the campaign with an objective. The first half of the season was extraordinary, probably above our capabilities. We had a run of five or six games where we stuttered and some of them we deserved to lose, some we didn’t.
“I said for five months there were teams better equipped to win and would emerge in the long run, which proved to be the case. We are still up there. We might be a little sad because we lost points stupidly or had a few games below par, but the objective does not change.
“We have to focus on ourselves. If we do what we can, then we’ve got a great chance of finishing third.”