The Spain captain feels that their opponents will know what to expect from the World champions, and laments the injuries of David Villa and Carles Puyol
The Spanish were convincing victors in both Euro 2008 and the 2010 World Cup, and have a chance to become the first ever team to successfully defend the Euros.
However, the Real Madrid star feels that his team-mates will have a tough task on their hands, but feels that they are capable of continental glory if they show the right desire.
"The national team has more than enough in our play. The main rival will be ourselves for the effort, the desire and the attitude that we have to win it," he told AS.
"We have the experience of winning the Euros and the World Cup, and we have to take advantage of this, although it will be very difficult to defend our title as champions of Europe.
"The other countries already know how we play, they don't need to study us. They are playing against the champions and they will be in top form to make the game their own.
Spain will be without Barcelona duo David Villa and Carles Puyol, who were both instrumental in 2008 and 2010, and Casillas admits that his nation have lost key players.
"The injuries to Puyi [Puyol], and El Guaje [Villa] have been a hard blow, but this team has players capable of occupying these positions and they can deputise.
"However, the feeling is that these are two very important injuries."
Spain kick off their Euro 2012 defence against Italy on June 10.