The Proteas already face the threat of bowing out of the tournament in the first round at the back of two comprehensive losses in the span of four days since the commencement of the tournament.
Du Plessis admitted committing a tactical error of electing to field first on a batting-friendly pitch on Sunday.
A daunting task now awaits South Africa as they will have to dominate the remainder of the games to secure a berth in the semi-finals.
They will be looking to register their first victory of the World Cup when they take on tournament favourites India on Wednesday.
Du Plessis promised a more firm approach in his captaincy hereafter, hoping to rectify the mistakes in the upcoming games.
"My style of captaincy has always been that there's a line and if you don't perform to that line there will be a lot of harsh words," he said.
"There's a time for strictness, and when you see that a dressing room needs you to be strong and to motivate them. But today was not good enough.
"If guys think they can make excuses for performances like today then they will be challenged - that's a fact. All I can say is that we'll keep fighting.
"I'm extremely disappointed and gutted to say that all formats of our game at the moment are just not firing.
South Africa’s World Cup campaign has also been unsettled by the unexpected bout of injuries to their key players, however, Du Plessis believes this aspect cannot be solely blamed for the defeats.
"To blame bad luck is not just an option for me. You still have to find a way to put in performances and we're just off in some areas."
Pacer Lungi Ngidi could manage to deliver only four overs against Bangladesh before suffering a hamstring injury, which reports suggest will rule him out for 10 days.
Du Plessis has cast an optimistic light on Hashim Amla’s fitness, who is expected to recover in time for the high-octane clash versus India in Southampton after he sustained a head injury in the showpiece event’s opener against England.
Veteran bowler Dale Steyn’s shoulder injury still remains a major point of concern as he has been ruled out of the tournament.
Du Plessis acknowledged that the team’s plans were not being perfectly executed. He pinpointed sloppy bowling with the new ball and the inability of batsmen to convert starts into considerable contributions as the main reasons for the defeat.
"Our plan in both the England game and this game was that we make sure we target them with aggressive bowling, but we weren't at our best," he said.
"The opening bowlers would be the first to say we didn't execute our skills today.
"We're just off in all facets and it's not good enough.
"With the bat once again we looked good, did some good things, but myself included, you need to go through and score hundreds, that's what wins you matches, not 30s and 40s."
Du Plessis did not rule out the possibility of South Africa becoming world champions for the first time and claimed the need to desperately try something out of the ordinary to get back to their winning ways.
"Plan A's gone. Now we have to look at all our options, reshuffling all our cards and seeing what we can do with it," he said.
"I have to believe we can still win the tournament. I wouldn't be South African if I said no to that.
"We know we're not good enough at the moment. I don't think it's got anything to do with identity. At the moment it's a skill thing.
"Every single player in our dressing room is not playing to their full potential.
"It's just about making sure you look at yourself in the mirror and see how you can find that answer."