During the 1977 Mexican League season, López earned a 19–3 win-loss record, a 2.01 ERA and 165 strikeouts over 157 innings in 73 games. Despite being a closer, he was fifth in the league in wins, and he set a new single-season save record (30). He was named the league's MVP. For several years in the mid- to late-1970s, López pitched winter baseball in the Mexican Pacific League. He broke league records by leading the league in games pitched three times, by earning eight consecutive relief wins in 1977–78, and by leading the league in relief appearances in four consecutive seasons. Among Mexican baseball fans, López became known as "El Buitre de Tecamachalco" ("The Vulture of Tecamachalco").