Blanco, the Giants' top outfield substitute since 2012, batted .225 in 101 games, a sharp drop from the .262 career average he brought into this year.
Blanco underwent tests on Monday that confirmed his injury, which he initially felt when he dove into first base in a May game.
"It's disappointing for me not to accomplish what I wanted to do," the 32-year-old Blanco said. He added that his swing lacked its usual "snap," which diminished his bat speed.
Said Giants manager Bruce Bochy, "I think [Blanco's injury] did start gradually and has been getting worse and worse to the point where he couldn't get a good swing off."
Hernandez, 28, was the choice to replace Blanco largely because Mac Williamson's strained left shoulder has continued to limit his playing time at Triple-A Sacramento. The Giants also wanted a right-handed batter to complement Denard Span when the left-handed-hitting center fielder needs a rest. That favored Hernandez while leaving the Giants inclined to keep lefty-swinging Jarrett Parker at Sacramento.
Hernandez's performance this season helped him, too. He batted .302 with 22 doubles, eight homers and 51 RBIs in 116 games at Triple-A.
Hernandez hit .186 in 78 Major League games in 2012 with Pittsburgh and Miami and '15 with Pittsburgh. Having ridden the Triple-A/big league roller coaster, he maintained a philosophical attitude about being called up before the end of this season.
"That's not my decision. If it happens, it happens," Hernandez said.
• The Giants announced that right-hander Chris Heston, their No. 5 starter in 2015 who has spent most of this season recovering from a left oblique injury, will begin his rehabilitation stint with Class A Advanced San Jose.