Du Plessis said that if the bowlers performed, it would be up to the top six in the batting order to make sure they scored enough runs.
England and India continue to hold the first and second place respectively despite the series defeats against West Indies and Australia.
Faf du Plessis and Quinton de Kock will be rested for the second and third T20Is of the series.
The win clinched a 5-0 series clean sweep for South Africa, continuing a horror run for the tourists.
Tahir, playing his final ODI in South Africa, scalped two wickets while Rabada got three wickets
Sri Lanka seek a consolation win while the Proteas eye a whitewash in JP Duminy's final ODI in South Africa.
With momentum firmly on their side, South Africa will use this match to finalise their playing XI ahead of the World Cup.
Lakmal, who last played a T20I in March 2018, while his last ODI appearance came in September in the same year.
Hathurusingha was stripped of his team selection responsibilities, in January, underscoring tensions within the team and its management.
Under Hathurusinghe, Sri Lanka have slipped in the ICC rankings and failed to secure an automatic qualification for 2020 T20 World Cup in Australia.