Previously unbeaten in 27 league matches stretching back into the 2019-20 season, Milan have now lost two of their last four games - they also went down 3-1 to Juventus earlier this month.
Goals from Cristian Romero, Josip Ilicic and Duvan Zapata condemned Stefano Pioli's side to their latest defeat, but they are still top at the midway point of the season as closest challengers Inter could only manage a 0-0 draw with Udinese.
However, Ibrahimovic accepts being winter champions will count for little unless Milan regain some consistency and end their decade-long wait for Scudetto success.
"We lost two games after more than 30 matches that we didn't lose," he told Sky Sport Italia.
"The team is a bit fragile because when certain players are missing, we lack a bit of experience. They are replaced by young players, but this is no excuse. "We make many sacrifices; we work and we are first for a reason.