Striker Mousset and midfielder Fleck have both resumed full training and are in contention to play a part in the game on Sunday. Mousset has yet to feature at all this campaign after suffering a tendon injury in a pre-season friendly while Fleck has been out with a back problem since the beginning of October.
Injury meant Enda Stevens did not travel to meet up with the Republic of Ireland squad and the defender is another player who will be given the opportunity to prove his fitness, along with Rhian Brewster and John Egan after both players left their respective international teams early.
The 30-year-old has missed the Hammers' last two games with a hamstring problem. He has been recovering during the international break and is back in training ahead of the trip to Bramall Lane.
Angelo Ogbonna is also working his way back from injury after he was forced off against Fulham with a knock two weeks ago.
Sheffield United provisional squad: Ramsdale, Foderingham, Baldock, Stevens, Egan, Basham, Lowe, Robinson, Bogle, Jagielka, Ampadu, Lundstram, Flack, Norwood, Berge, Osborn, McBurnie, Sharp, McGoldrick, Burke, Brewster, Mousset.
Provisional West Ham squad: Fabianski, Balbuena, Ogbonna, Cresswell, Coufal, Masuaku, Rice, Soucek, Bowen, Benrahma, Haller, Randolph, Fredericks, Diop, Snodgrass, Lanzini, Fornals, Yarmolenko, Antonio.