The summer signing missed last weekend's 2-1 loss away at Everton through injury after coming on as a 56th-minute substitute in the opening-weekend draw with Burnley.
However, Elyounoussi is back and in line for his full Saints debut following a swift recovery, with Charlie Austin the man to miss out against the former Premier League champions.
Southampton: McCarthy; Soares, Vestergaard, Hoedt, Bertrand; Hojbjerg, Lemina, Elyounoussi, Redmond; Long, Ings
Subs: Gunn, Stephens, Davis, Austin, Romeu, Ward-Prowse, Armstrong
Leicester City: Schmeichel; Amartey, Chilwell, Morgan, Maguire; Mendy, Ndidi; Pereira, Gray, Maddison; Iheanacho
Subs: Ward, Evans, Albrighton, Okazaki, Silva, Fuchs, Ghezzal