Guts made his professional debut in 1966. He was known primarily as a rough, undisciplined fighter early in his career, but his technique improved greatly under the tutelage of American trainer Eddie Townsend. He challenged Panamanian Ismael Laguna in 1970 for the WBA/WBC Lightweight Title, but lost by TKO in the 13th round. He challenged Shinichi Kadota (who had knocked out Guts only five months earlier) in 1972, and won by decision to capture the OPBF Lightweight title. After the fight, he remarked that his goal was to fight the WBA/WBC champion Ken Buchanan. Buchanan would challenge Guts three years later, when Guts was the WBC Lightweight champion.