Hernández signed with the Philadelphia Phillies as an amateur free agent in 1973. He was selected by the Chicago Cubs from the Phillies in the 1976 Rule 5 Draft. In his first two years in the majors, Hernandez went 16–9 with a 3.03 ERA in '77 and a 3.77 ERA in '78. The Cubs gave him a try as a starter in 1980, but he went 1–9 that year in 7 starts and 53 games pitched. From 1979 to 1982, Hernández was 9–19 and gave up 128 earned runs in that time. After being traded to the Phillies for Bill Johnson and Dick Ruthven, he went 8–4 with 3.29 ERA and 7 saves. The Phillies made the World Series that year, losing to the Baltimore Orioles in 5 games. Prior to the 1984 season, Hernández was traded to the Detroit Tigers with Dave Bergman for Glenn Wilson and John Wockenfuss. He had a 9–3 record, 32 saves and a 1.92 ERA in 140 innings pitched, while recording 112 strikeouts. In the entire season, Hernandez gave up only 6 home runs, 30 runs, 36 base on balls, and 96 hits. His 32 saves came in 33 opportunities. Hernández won the 1984 American LeagueCy Young Award, beating out Dan Quisenberry. He also became one of the few relievers to ever win the Most Valuable Player Award, edging out Kent Hrbek. The Tigers finished 104–58 that season, and went on to sweep the Kansas City Royals in the American League Championship Series and defeat the San Diego Padres in the 1984 World Series, four games to one. Hernandez pitched five innings in the series, had two saves, and gave up only one run. His overall postseason stats include a 2–2 record and a 1.32 ERA in 10 games, innings pitched. He allowed 9 hits, walked 2, and struck out 7. Hernández never had those numbers again, but was a solid reliever for the Tigers in his final years - when he became known by his given name Guillermo Hernández. In 1988, his second-to-last year, he went 6–5 with a 3.06 ERA and 10 saves in 67 innings pitched. Hernández's last MLB game was on August 18, 1989, before being released by Detroit. He attempted comebacks with the Triple-ASyracuse Chiefs of the Toronto Blue Jays organization in 1991 and Columbus Clippers of the New York Yankees organization in 1995. As a hitter, Hernández posted a.206 batting average with 3 RBI. Defensively, he was better than average, committing only four errors in 231 total chances in 1044.2 innings pitched for a good.983 fielding percentage.
