The Zebrette won 3-1 at the San Siro on Saturday.
"It was a bit unexpected, the start of the second half, especially as we'd done well in the first half. We had the right distances, shape and attitude. We didn't finalise our chances as well as usual, but we created them," Spallettu told Sky Sport Italia.
“In the second half, we made too many simple errors and Udinese managed to create this two-goal cushion. I tried to introduce Roberto Gagliardini to reduce the gap in physical strength, but it didn't change anything.
“This must teach us that when you lower the intensity in a league with such narrow margins, you will drop points. Even the tiniest situations can make the difference."