Janse van Rensburg injured her ankle in Saturday's 41-29 win over Italy, while Malinga is struggling with a knee injury sustained in Wales a week ago.
Byrhandre Dolf, who started the Italy match at flyhalf but left the field on Saturday for a head injury assessment, will complete her HIA 3 protocols on Monday, after which a call on her availability for the WXV Global Series Test against Spain will be made.
Zulu, who last played for the Springbok Women in the Rugby Africa Women's Cup in Kenya in May, will join the squad on Monday to provide flyhalf cover.
The 23-year-old Zulu made her Test debut against Spain in Madrid and was a member of the squad that played at the Women's Rugby World Cup in England last year. She has played in 18 Tests, scoring 45 points, and also captained the South African Women's 'A'-side to Brazil last month.
The match against Spain in Seville on Sunday will be the final match in the three week WXV Global Series tour for the Springbok Women, who will also take on Ireland next month in a two-Test series in Cape Town to conclude their 2026 season.
The Bok Women will start their preparations for the Test against Spain on Monday.