Bento expects Ronaldo to be decisive
The Euro 2004 runners-up were the first side drawn in Group I before being joined by Denmark, Serbia, Armenia and Albania in the only five-team group.
First and second from all nine groups are guaranteed passage to the finals in France, while the best third-placed side will also go straight through.
The remaining eight teams to finish third will battle it out in the play-offs and, although Bento believes Serbia and Denmark will prove worthy adversaries, the former Sporting Lisbon defender thinks he holds the ace in the pack in the form of Real Madrid star Ronaldo, winner of the 2013 Ballon d'Or.
It was Ronaldo's brilliance that saw him score all four goals as Portugal defeated Sweden 4-2 on aggregate in November in the play-offs to reach this year's World Cup.
Bento said: "It is a difficult group, I have to say. The level of the top national teams is similar.
"It is very equal and it will be tough, but we have one point in our favour; we have the best player in the world (Ronaldo).
"Cristiano works well at everything. He is a fantastic professional and now he is an ambitious player.
"He always wants to be a better player. It is incredible, but he is still improving as a player and everybody expects a lot from the best player in the world."
Portugal have failed to convince in each of their last two European Championship qualification campaigns, finishing second to Poland before Euro 2008 and Denmark for Euro 2012, and Bento insists the current crop have to put that right this time around.
He added: "The last two qualifications were not so good, but this time it is different because the first two teams qualify directly and the best third placed.
"It is important to have a better qualification than the last Euros. It is difficult to say (who will pose the biggest threat), but Denmark, Serbia and Portugal are the strongest teams (in the group).
"Between these three will be the qualifiers. It will be equal, not easy. It is a tough group."