The former Formula 1 supremo does agree, however, that the possibly Indycar-bound Spaniard is one of the "best drivers" ever seen in F1.
"He is definitely one of the best drivers, but sometimes he made the wrong decisions. He is certainly not the greatest in other areas of life," the 87-year-old is quoted as saying by Bild.
"Stirling Moss was never the world champion, but he was one of the greatest."
However, Alonso is still revered by F1 fans around the world, and especially in his native Spain.
"I'm not afraid of that," Sainz told AS.
"There are a lot of Formula 1 fans thanks to Fernando. It is true that many F1 fans came from Spain first for Fernando but they stayed because they like F1 as a sport.
"Of course there will be people who go with Fernando to Indy, but I think it is thanks to Fernando that there is a very large fan base in Spain. I will do everything I can to keep it that way."
Alonso currently sits 11th in the 2018 driver rankings.