Football provided him with speed and exceptional footwork, skills that would later contribute to his development as an outstanding basketball player.

He led his high school, St. Clement's Academy of La Paz, Iloilo City, to the PRISAA basketball championship.

[1] In 1971, he began playing in the Manila Industrial and Commercial Athletic Association (MICAA), first with the Meralco Reddy Kilowatts in 1971, the U/Tex Weavers in 1972 and the Toyota Comets in 1973.

Arnaiz was one of the original members of the Toyota franchise that joined the newly formed Philippine Basketball Association (PBA) in 1975.

After Toyota disbanded in 1984, Arnaiz followed Jaworski in a controversial move to Gilbey's Gin (now known as Barangay Ginebra San Miguel).

In 2004, during one of Arnaiz's visits to the Philippines, he was interviewed by various media outlets recalling several memories from his playing days to his life after retirement from basketball.