Seam-bowling allrounder Lahiru Madushanka and wicketkeeper-batsman Sandun Weerakkody received their maiden call-ups in ODIs and are due to join the squad already in South Africa along with left-arm spinning allrounder Chaturanga de Silva, who has played six ODIs.
They will act as cover for fast bowler Nuwan Pradeep and opener Danushka Gunathilaka who have returned home with hand and back problems respectively.
In November, when both Mathews and Chandimal were injured, Tharanga became Sri Lanka's 19th captain in ODIs, and led them to a tri-series win in Zimbabwe. This came after Tharanga was dropped for the home series against Australia, only playing in the fifth ODI after the series was lost.
Chaturanga last played for Sri Lanka in 2014, but he has worked his way back into contention with strong performances in first-class cricket. He picked up 35 wickets in six matches at 21.71, including two five-wicket hauls, in addition to 560 runs at 50.90, including six fifties, for Nondescripts Cricket Club in the Premier League tournament.
Madushanka hit 346 runs and took 15 wickets in seven matches in the same tournament, while Weerakoddy, opening the batting, scored 530 runs in six matches. He is the third wicketkeeper-batsman in the squad behind Nondescripts Cricket Club team-mates Chandimal and Niroshan Dickwella.
The first ODI begins in Port Elizabeth on January 28.