Riedewald replaces left-back Patrick van Aanholt, who was substituted against the Hammers with an apparent strain. In midfield, McCarthy takes the place of Cheikhou Kouyate, who is named on the bench today.
Also on the bench are Development prospects Sam Woods, Brandon Pierrick and James Daly makes a return to the squad as a potential left-back or forward after making the 18 against Newcastle United.
The Saints also make two changes, with in-form Danny Ings replacing 19-year-old Michael Obafemi, who netted against Chelsea. Ralph Hasenhüttl stated pre-match that he would not start the young forward due to the density of recent matches and a propensity for injuries.
Ings has scored seven goals from his last eight appearances.
Their other switch sees Sofiane Boufal recovered from a recent toe injury and start in place of Stuart Armstrong, who takes a place on the bench. Substitute centre-back Kevin Danso has been replaced by 19-year-old Will Smallbone and Moussa Djenepo fills the space left on the bench by Ings receiving a start.
Southampton: McCarthy, Cedric, Stephens, Bednarek, Bertrand, Ward-Prowse, Hojbjerg, Boufal, Redmond, Adams, Ings.
Subs: Gunn, Yoshida, Vestergaard, Djenepo, Romeu, Armstrong, Smallbone.
Palace: Guaita, Kelly, Tomkins, Sakho, Riedewald, McArthur, Milivojevic, McCarthy, Meyer, Ayew, Zaha.
Subs: Hennessey, Woods, Kouyate, Daly, Camarasa, Pierrick, Wickham.