January signing Soucek suffered a hip injury at Liverpool on Monday night and faces three weeks out, while Fredericks had a shoulder operation this week.
Jarrod Bowen could make his full debut after two substitute appearances since his deadline-day switch from Hull.
Kyle Walker-Peters is back in contention for Southampton's trip to the London Stadium.
The Tottenham loanee has shaken off a calf concern and has trained all week with the south coast club.
Nathan Redmond remains sidelined with a hip injury and could be out of action for another month.
West Ham provisional squad: Fabianski, Ngakia, Diop, Ogbonna, Cresswell, Rice, Noble, Snodgrass, Bowen, Anderson, Antonio, Randolph, Zabaleta, Balbuena, Masuaku, Lanzini, Fornals, Haller.
Southampton provisional squad: McCarthy, Valery, Walker-Peters, Stephens, Vestergaard, Bertrand, Ward-Prowse, Hojbjerg, Armstrong, Ings, Long, Adams, Djenepo, Romeu, Obafemi, Smallbone, Gunn, Bednarek, Vokins.