With four wins and a draw from their first five Group C qualifiers, Italy are well-placed to qualify for the tournament in Poland and Ukraine but Prandelli remains determined to stamp his authority on the new-look Azzurri.
"From August onwards anybody not playing regularly will find it hard to get into the squad," Prandelli told reporters as the team prepared for Friday's fixture in Modena at the national team training centre in Tuscany.
"We'll be closer, I hope, to an important event and we must be ready for it. This is a general point, I'm not talking about individual players."
Milan's Antonio Cassano, who scored one of the goals in the 2-1 comeback win in Estonia last September, is likely to start up front alongside former Sampdoria team-mate Giampaolo Pazzini while Mario Balotelli is also in the squad despite having fallen foul of Prandelli's new disciplinary code.
The coach said both needed to be on their toes.
"I've repeated to Balotelli what I said before," said Prandelli of his advice to the Manchester City striker to calm down after he was sent off in European competition this season.
"I think he's understood me, he's not pleased with his season despite its happy ending. [Antonio] Cassano will have to force his way into the AC Milan team. People who don't play much generally risk losing confidence."
Roma's troubled Daniele De Rossi is absent from a midfield likely to feature Juventus pair Alberto Aquilani and Claudio Marchisio, Fiorentina's Riccardo Montolivo and Inter Milan's Thiago Motta, who scored the winner in March's 1-0 win in Slovenia.
"I'm sorry to have to say this, but De Rossi has not been left out for technical reasons," Prandelli said of De Rossi, who was recently banned for three games for elbowing in a Serie A match against Bari.
"He'll have to get back the consistency and concentration he seems to have lost in recent months.
"The door will always be open for Daniele, but there are forms of behaviour on the field I just won't stand for, and I've been saying this since day one."
Prandelli said Estonia could be Italy's toughest test since he took over the Azzurri hot seat following last year's World Cup failure.
"Estonia is going to be the decisive game," he said of the Eastern European side who have picked up four points from Serbia in their last two matches. "I want a group prepared to graft. We've always struggled when we play in June and September."
Estonia, who are six points behind Italy after five games, travel to Modena with a depleted squad after influential midfielder Aleksandr Dmitrijev was ruled out through injury and Konstantin Vassiljev, their top scorer in qualifying, misses the game through suspension.
In another blow to coach Tarmo Ruutli, forward Andres Oper and defender Andrei Sidorenkov are both injuryed with Taijo Teniste and Sergei Mosnikov in contention to make their international debuts.
Middlesbrough's Tarmo Kink, who scored in the impressive 3-1 win in Serbia, is fit though as Estonia, a point behind Slovenia and Serbia but wi
