Six points separate La Viola from third-placed Milan, meaning that a home defeat at the Stadio Artemio Franchi could leave Montella's men nine points adrift.
But the former Roma striker witnessed his old side go down 2-0 at relegation-threatened Palermo last weekend and used the example to warn the Rossoneri to be on their guard on Sunday.
He told La Repubblica: "The Italian league is extremely difficult and there are always surprises around the corner, just like Palermo proved by beating Roma.
"We must not let our heads drop and we must rediscover our best football and the best of Fiorentina.
"There are a lot of different games between now and the end of the season and our objective hasn't changed."
Giuseppe Rossi has received the all-clear from the latest follow-up check on a cruciate ligament injury but the ex-Parma striker remains weeks away from a return to action.
Fiorentina are also without Stevan Jovetic, who will miss two weeks with a leg problem, with Michele Camporese and Ahmed Hegazy also sidelined.
Winger Juan Cuadrado said of his Montenegrin team-mate: "He's someone who gives the team a lot and we will definitely miss him.
"But there are many other strong players who want their chance to shine. We are expecting a tough game against Milan but we will do our best.
"There are three very important points up for grabs."
Milan striker Giampaolo Pazzini travelled to Florence with the squad and could feature following an ankle injury, but the season is over for Holland midfielder Nigel De Jong.
Italy forward Stephan El Shaarawy shrugged off suggestions his form has dropped since Mario Balotelli arrived at San Siro in January.
The 20-year-old has 15 league goals to his name this season but only one has arrived since Balotelli made his debut against Udinese on February 3.
But he told Gazzetta.it: "These are baseless criticisms. He and I understand each other's game perfectly.
"In every game where we've played together we've managed to do good things - whether goals or assists. I think our performances in the national team explain the situation better."
Recent history is in the Tuscans' favour, with Milan taking only one point from three meetings since their last victory back in April 2011.