'If they come off it is going to make it easier for the likes for Kane and Rosco and the guys after them,' says New Zealand's coach Stead.
Rabada's fitness is a worry after he had to withdraw from the Indian Premier League this year because of a back strain.
The visitors chase down their 278 target with 10 balls to spare, inflicting the first one-day defeat on Australia in 10 games.
India trail the five-test series 2-1 and need to win to keep alive their hopes of a series victory.
Bairstow's place in the side was in doubt after the 28-year-old was hurt while keeping during the third test.
England ended their second innings 317 all out after Jos Buttler’s maiden test century on Tuesday provided some cheer in what was a poor batting display against fine Indian seam bowling.
The next wicket taken by Dale Steyn will give him South Africa's national record, with 422.
Rabada, who took 4-41, inflicted the early damage while left-arm spinner Shamsi provided the late breakthroughs with his 4-33 as South Africa bundled out Sri Lanka for 193 under 35 overs.