We know that Allegri is a coach who doesn't take the risk of doing things he doesn't feel are within his capabilities. He's certainly a very conservative coach, but if the players follow him and everyone does what he says, we'll surely see an excellent team. It's impossible for them to reach the top of the league, but they can certainly do well.
Milan's victory at Lecce was a natural outcome, as Milan has been making improvements under Allegri's guidance this season, and the Milan players are certainly well aware of that. However, reaching the top of the league table, I believe, will still be very difficult for Milan, as there are still several other strong teams that will stand in their way. Furthermore, Milan's ability this season hasn't surpassed that of Inter and Napoli, so reaching the top of the league isn't something to be expected from AC Milan this season.
With this first win, at least AC Milan can have better confidence, giving them a chance to top the standings. Lecce is basically not a worthy opponent for AC Milan since they are a mediocre club. The strength of the two teams is certainly not balanced, so I don't think AC Milan strength this season has been fully tested. I think AC Milan true strength can only be seen when they face fellow elite Serie A teams.
However, considering AC Milan is only focused on domestic competition, Allegri job should be a bit easier to get his team back on track. Last season, AC Milan finished eighth, Allegri should be able to guide AC Milan to a top four finish and secure a Champions League spot when the season ends.