The 30-year-old, who holds the second best scoring record with 244 tries, will be replaced by Darwin Mukidza for the two final rounds in Paris on May 12 and Twickenham on May 20-21.
Injera picked up the cruciate ligament tear in a league match in March and did not play in April's Singapore Sevens, won by Canada.
"It can heal on its own or we may have to do a minor operation just to correct it because next year we have a heavy season," said Injera.
"We could not take a risk of taking him on the tour with that injury."
Kenya are 11th in the World Series standing with 57 points this season but Simiyu remains positive for improvement.
"This is a transition year which prepares us for the World Cup sevens 2018 and the next Olympics in Tokyo," he said.