Hernandez's batting average hovered around .250 for most of his minor league career, until his promotion to the Tulsa Oilers in the second half of the 1973 season. With the Cardinals' AAA affiliate, Hernandez batted .333 with five home runs and a .525 slugging percentage. The following season, Hernandez's average jumped to .351, earning him a promotion to the big league club. He made his major league debut at Candlestick Park on August 30, 1974, against the San Francisco Giants, going 1-for-2 with two walks, and earning his first major league RBI with a single in the ninth. Following the season, the Cards traded first baseman Joe Torre to the New York Mets for Tommy Moore and Ray Sadecki to make room for their budding young prospect.