Allegri has agreed to give veteran Gianluigi Buffon a rest but has challenged his fringe players to keep up Juve's 100 per cent start to their title defence.
"After the game with Barcelona [in the Champions League] I decided, together with Gigi, to play Szczesny in this game," he told a news conference. "That doesn't mean I have more or less confidence because Neto played a lot of games last season.
"Szczesny is a very good goalkeeper, one of the best in the world, so for Juventus to have two goalkeepers like Buffon and Szczesny is something really important.
"So Buffon will be rested and he'll be on the bench. Physically we're okay but we have another important game on Saturday [against Torino], then Wednesday again and then Sunday.
"The important thing is that the team, even when there are changes, doesn't suffer a dip. The most important thing is to arrive at the crucial phase of the season in the best shape."
Juve beat Fiorentina 2-1 in Turin last season but lost the reverse fixture by the same scoreline.
Allegri believes it will be difficult to predict results against the top sides in Serie A after last year's inconsistencies but has already given his side a points target as they seek a seventh title in a row.
"It's important to win games against the so-called medium or small teams, because they're important points," he said. "Then, in the direct meetings, anything can happen.
"The strange thing is that, last year, I think there was only one draw in all the direct clashes, us and Napoli, then in all the others one team won.
"What's certain is we'll need 90 points to win the league."