Johnson, 21, earned a rookie contract this year after impressing with Premier Cricket side West Torrens Cricket Club.
He is a left-arm fast-medium bowler who was named the Rob Zadow player of the match in the West End One Day Cup final against Glenelg for his 5-40.
Johnson took three wickets in four balls for the Western Grit against the Southern Force which won his side the match on the weekend, restricting the Force to two runs in the final over when they needed 12 to win.
Mennie is eligible to return to the side following the birth of his child.
The Redbacks face the Victorian Bushrangers at North Sydney Oval tomorrow from 10:30am (local).