The Viola recorded a remarkable 4-3 win over Torino at the Artemio Franchi during the afternoon, while the Rossoneri lost 1-0 to league leader Juventus in the evening kickoff in Turin.
With five rounds of the season remaining, Italy's third and final Champions League berth is clearly now well and truly up for grabs, but Allegri is backing his side to keep Fiorentina at bay.
"We are not worried," the Rossoneri boss told Sky Sport Italia. "We have lost our first game of the year. Now we must try to do well in the next games to hold onto third place.
"We know that Fiorentina are a good side. We threw away a match point by not winning in Florence despite being 2-0 up. Now our calendar becomes easier. I am confident."
However, Allegri was still left counting the cost of Milan's misfortune in Turin, with the Rossoneri losing both Christian Abbiati and Massimo Ambrosini to injury before gifting the home side the game-winning penalty with a defensive mix up between Ignazio Abate and Marco Amelia.
"It was an evenly balanced game in which we conceded very little to Juve," the former Cagliari boss mused. "It was a game that seemed destined to end in a draw, but then we were punished for our only error in defense.
"Unfortunately we have also lost two players to injury and even [Riccardo] Montolivo was not at his best, physically."
Allegri was keen, though, to defend top scorer Stephan El Shaarawy, after another lackluster display from a player who played brilliantly during the first half of the season.
"He's a boy of 20 that has done very well up until now, but it's normal that there can be a dip [in form]," he argued. "Physically, he's fine, but he must understand these difficult moments can happen."
Allegri is in no doubt that El Shaarawy will bounce back and he firmly believes that Milan will also benefit from its experiences this term.
"The true value of Milan will be seen next season," he claimed. "Then we will be know if we can compete with Juve.
"We have had a great second half of the season; but I hope we don't undo all that in the remaining games."
Milan hosts Catania next weekend, while Fiorentina travels to Genoa to take on Sampdoria.