They suffered another serious setback against Bangladesh when pace bowler Lungi Ngidi went off with a hamstring injury early on and Du Plessis said South Africa would only cope if they changed tactics in their remaining games.
"I'm extremely disappointed," the 34-year-old told reporters. "Our plan in both the England game and (the Bangladesh) game was to try and make sure we target them with aggressive bowling.
"Plan A is gone because Plan A was those main bowlers playing together and they haven't played a game together on this tour.
"So now we have to really look at what we can do to try to be effective -- is it playing all-rounders together, do we play two spinners? Now it's reshuffling all our cards to see how best we can deal with it."
Steyn is expected to be fit for Wednesday's game against India and Du Plessis said every member of the squad needed to start living up to their potential.
"I have to believe that we can still win the World Cup," he added. "I won't be South African if I said no.
"Every single player in our dressing room is not playing to their full potential and that's why we're not putting the performances in. As soon as that starts happening, then those things will change.
"We're playing a strong team in India, and as a team, we know we're not good enough at the moment and we have to turn it around."
(Reporting by Shubham Kalia in Bengaluru; Editing by Simon Jennings)
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