On June 17, Hernández became the thirteenth AL pitcher ever to throw an "immaculate inning" (striking out the side on exactly nine pitches). He did this versus the Florida Marlins in the 4th inning. On June 23, in his only at bat of the season, Hernández hit his first major league home run, a grand slam, off Johan Santana of the New York Mets. Hernández became the first American League pitcher to hit a grand slam since Cleveland's Steve Dunning hit one on May 11, 1971, and was the first to do so since interleague play and the DH rule began. It was also the first home run ever hit by a Mariners pitcher. Later on in the same game, Carlos Beltrán slid into Hernández as he covered home, injuring the pitcher's left ankle. He was forced to leave the ballgame and was placed on the 15-day disabled list. He finished the season with a perfect 4.000 slugging percentage.