Arnaiz was one of the original members of the Toyota franchise that joined the newly formed Philippine Basketball Association (PBA) in 1975. Along with Robert Jaworski and Ramon Fernandez, he formed the troika of the vaunted Toyota offense. In fact, among the three players, he would have been the first to have won the Most Valuable Player (MVP) award right in the very first season of the league in 1975 when media covering the PBA named him as the Basketball Player of the Year despite the fact that Bogs Adornado was the one named PBA MVP that year. During his Toyota years, the franchise won nine PBA titles while having memorable battles against arch-rival Crispa Redmanizers, going head-to-head, on and off the court, with Crispa's Bernie Fabiosa. Arnaiz became one of the most popular players on the team not only for his playing style, but also for his "mestizo" looks, which gained him a following among the female fans.