Steyn was ruled out of the World Cup due to a second shoulder injury while fellow quick bowler Ngidi is still recovering from a hamstring problem.
South Africa, who lost to England and Bangladesh, are looking for their first win of the tournament on Wednesday against India who are ranked second in the world.
"There's a lot of injuries," Du Plessis told reporters during a training session at Southampton. "Zero from two."
"Naturally, guys will have a little bit of confidence that's been chucked away and that is normal, that is part of being human.
"But it's just making sure that we keep strong, keep fighting and stay true to ourselves as a team and a culture."
South Africa have yet to win the 50 overs world title and Du Plessis said the options are limited with a depleted bowling attack against India who are playing their first game of the tournament.
"Either you decide if you want to go with all your all-rounders in one team, and have a long batting line-up and try and change your game plan a little bit and bat up until nine and hopefully you can get a big score, chase anything because you have a lot of batting or the two spinners," he said.
Steyn, 35, was injured during the Indian Premier League and missed the first two World Cup games. He will be replaced by left-arm paceman Beuran Hendricks in South Africa's squad for the rest of the tournament.