"I've already said that I think what he's doing shows that he's a real driver," Villeneuve, the 1997 world champion and a past Indy 500 winner, told Spanish daily AS.
Asked what he means by 'real driver', the French Canadian explained: "Well, all you have to do is listen to what other drivers have said about his decision.
"I heard Nico Hulkenberg and Romain Grosjean giving their opinion about what Alonso is doing - Hulkenberg said it's dangerous, that he wouldn't do it, that he did Le Mans because it was different.
"It sounds like excuses. I think there are two classes of drivers. You're either an Alonso or you're a Rosberg. And Hulkenberg is a Rosberg."
Next up on the F1 calendar is the Russian Grand Prix next weekend.