Alonso's former club Real Madrid have won the competition for the past two years, eliminating Napoli on their way last term, but the 35-year-old expects Maurizio Sarri's men to challenge their dominance.
The Spaniard lauded Sarri's side as "attacking and fearless", with Napoli – who finished third last season – having kicked off the campaign with a 4-0 aggregate win over Nice in the Champions League play-offs, and consecutive 3-1 victories in Serie A.
"I really like this Napoli. I like Sarri's philosophy and the desire of his team to always have the ball at their feet," he told Sky Sport Italia.
"Napoli are attacking and fearless. They have a clear mentality on the pitch and I appreciate it. I also have many friends there: Pepe Reina, [Jose] Callejon and [Raul] Albiol.
"I often hear from them and I like to watch them play in Napoli’s system. As well as in Italy, they can be a surprise package in Europe."
"Juve are a team among the favourites [for the Champions League]," said the 35-year-old. "They played two Champions League finals in three years and it's no coincidence.
"They're very close to the top, and sooner or later they'll end up winning it. I know how much the players want it, but there's so much competition.
"I was really close to Juve. I'd have liked to play in Italy but there was no agreement between Liverpool and Juve."