Santner was preparing for the tour at New Zealand Cricket's High Performance Centre in Christchurch when he was struck on the thumb fielding a ball off his own bowling on Monday. Coach Mike Hesson said it was unfortunate timing for Santner, who had impressed during his first tour with the national side to England this year.
"We feel for Mitch because he was really excited about this tour and we were looking forward to seeing him continue his development in international cricket," Hesson said. "It is unfortunate but at the same time it presents George with his first opportunity in the Black Caps environment and we are confident that he'll fit in well."
Worker, 25, offers similar skills to Santner, as a left-hand top-order batsman and left-arm orthodox spinner. He was the second-leading run scorer in the Georgie Pie Super Smash in 2014-15, with 318 runs at 45.42, and in addition to his outstanding run tally in the shorter formats he also collected nine Ford Trophy wickets at 20.11.
Anton Devcich was unavailable for consideration as he continues to recover from a shoulder operation. New Zealand will be led on the tour by Kane Williamson, who will captain the side with Brendon McCullum being rested. The tour includes three ODIs and one T20 against Zimbabwe in Harare, followed by two T20s and three ODIs in South Africa.