Luciano Spalletti apologised for his first defeat as Inter coach and called for his side to go back to basics in their Serie A title bid.
Inter led the league table heading into the weekend, but a costly 3-1 home defeat to Udinese on Saturday handed opportunities to second-placed Napoli and third-placed champions Juventus to take top spot.
Spalletti admitted the stunning defeat was a strange feeling for his previously unbeaten players, but added that it is important they return to the principles rooted in their title challenge.
"I'm sorry for this defeat - like all of them," he told Mediaset Premium. "It had been a long time and we had forgotten how it feels.
"In the second half we were not able to keep the ball and we made mistakes, also not making brave choices. It did not turn out so well. This is something new, especially what happened to us at the beginning of the first half.
"Now we have to go back to doing the things that brought us to this point."
18 - Today's defeat ended Inter's 18-game unbeaten run in Serie A (between this and the last season). Stop. #InterUdinese-- OptaPaolo (@OptaPaolo) December 16, 2017
Assessing the title race, Spalletti acknowledged that this was a huge blow as a number of sides maintain a fine pace at the top.
"For the teams at the top, it is a continuous sprint and it is clear that those who stop for a moment are overtaken," he added. "We obviously did not want this.
"With this small mistake, the other teams have gained a great advantage."