Sean is nursing a knock while younger brother Matty, who is currently in contract talks amid interest from Udinese, has a thigh problem and neither is expected to be fit.
Striker Dwight Gayle is out with a knee injury and midfielder Matt Ritchie is a doubt with a knock, but goalkeeper Martin Dubravka is available after recovering from a knee problem which looked to have ended his season, while Danny Rose, Valentino Lazaro and Nabil Bentaleb have had their loan deals extended.
Central defender O'Connell missed Wednesday night's 0-0 draw at Aston Villa along with midfielder Fleck after both picked up training-ground knocks, but Wilder is keeping his fingers crossed ahead of the fixture on Tyneside.
Striker David McGoldrick, who landed awkwardly after coming on as a substitute at Villa Park, also has a chance of making the squad.
Newcastle provisional squad: Dubravka, Darlow, Yedlin, Rose, Lazaro, Manquillo, Lascelles, Fernandez, Schar, Lejeune, Bentaleb, Hayden, Shelvey, Ritchie, Allan, Saint-Maximin, Almiron, Joelinton, Muto.
Sheffield United provisional squad: Henderson, Moore, Egan, O'Connell, Robinson, Basham, Baldock, Lundstram, Stevens, Berge, Fleck, Norwood, McBurnie, Sharp, L. Freeman, Mousset, Clarke, Jagielka, Rodwell, Osborn, K. Freeman.