Barcelona welcome Chelsea to Camp Nou for the first leg on Tuesday, with Belletti returning to his former club as part of Guus Hiddink's squad.
The versatile Brazilian spent three years as a Barcelona player and believes the last four encounter is worthy of being the final itself.
He expects the return leg at Stamford Bridge to be the decisive meeting and outlined Chelsea's desire to win the Champions League after last season's penalty shoot-out defeat by Manchester United.
Nobody unstoppable"I think the winners will end up being champions," Belletti told Barcelona's official website. "Barca have beaten some of the great teams of Europe, and we've beaten Juventus and Liverpool. So whoever picks up the cup will have defeated the best in Europe.
"I think people thought this would be the final. Obviously there are other great teams like Liverpool and Manchester United, but Barca and Chelsea are the two best teams in their leagues at the moment. It's a game worthy of the final.
"Nobody is unstoppable. I've played in the Brazilian national side for a long time and we've never thought we were unstoppable. When I was with Barca we played great football but we weren't either. Unstoppable teams don't exist.
"They play at a high level both on and off the ball. In my Barca days, I remember we put on the pressure when we lost the ball. They still do this now and the Chelsea players are aware of it.
Desire"But I've never been with a team like Chelsea, a team that wants to win more. Some of the players have won just about everything, but they never give up fighting.
"We have some great individual players, but we are also a great team. Didier Drogba, Frank Lampard and John Terry are on great form, and Nicolas Anelka is regaining his form.
"What makes us a strong side is our desire to win the Champions League. Our weaknesses depend on the day. We're capable of going to Camp Nou and not losing.
"I think the second leg will be decisive. In the first leg you still know you have another 90 minutes. It would be great to score in Nou Camp, but we also realise we can't afford to let any goals in. We want the semi-final to be decided at home."