Arnaiz was one of the original members of the Toyota franchise that joined the newly formed Philippine Basketball Association 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 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. He was given the moniker "Mr. Clutch" for his propensity to hit clutch baskets during crucial moments of a game. He is known for looping layups against imports and local behemoths that were impossible to block, and booming outside shots. After Toyota disbanded in 1984, Arnaiz followed Jaworski in a controversial move to Gilbey's Gin. Reprising their old backcourt partnership in Toyota, the duo turned the Gins into a perennial contender. However, he was sidelined due to a knee injury and was placed in the injured list but, was part of the line-up, during the early part of 1986 Open Conference when Ginebra San Miguel won its first PBA title. He quietly retired from basketball shortly afterwards. He won 10 championship in his illustrious PBA career, 9 with Toyota and 1 with Ginebra San Miguel. Arnaiz is considered as one of the most explosive guards in the PBA. He was a member of both the 5,000 and 10,000 points club having finished his career with a total of 10,292 points for an average of 16.8 points per game. He was also a member of the 2,000 assists club. He was the other half of the dreaded Batman and Robin tandem of Toyota, the other being Jaworski himself.
Retirement
After his playing career, Arnaiz immigrated to the United States, resided in California. He worked for the California state government while also becoming an evangelist. Arnaiz claims in an interview that his life abroad was quite different from his life in the Philippines. 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. He was supposed to play in 2003 Crispa-Toyota Reunion Game and the 2005 PBA Greatest Game but failed to do so due to his prior commitments. Arnaiz was named as one of the PBA's Greatest Players and was inducted into the PBA Hall of Fame in 2005.
Quotes
from Rudy Salud, former PBA Commissioner - "He fired up the fans with his looping layup. He had that style, and looks, and movement. That fired up the fans."
from Leo Prieto, the first PBA Commissioner - "He was a very fast player and made some moves that were exceptional for a small player. Because of his football background, he had the legs to compete, to go over the defensive player."
on his PBA career - "I was not like Sonny while I was still playing basketball in the PBA. I took so many things and people for granted. Despite my career records and many championships, I never tried to be the best I could be. I never gave the game my all. And that is so sad. Looking back, I realize today that championships, trophies, achievements, and records are all so temporary. Sooner or later, someone else or another team will win the same championships, break those records and win the same trophies. They are never permanent. Ultimately, what no one can take away from you is when you can say to yourself at the end of the day, win or lose that 'I did my best.'"
