Hayes, the former Barnsley and Scunthorpe striker, signed for Newport in October, but registration issues were not resolved until this week.
County welcome back central defender Mark O’Brien after a three-game ban, while Sean Rigg and Calaum Jahraldo-Martin are available again after hip and hamstring injuries.
But Marlon Jackson (hamstring) remains out and Aaron Collins and Aaron Williams are both cup-tied.
Leeds pair Ronaldo Vieira and Stuart Dallas could both return to the starting line-up after injury.
Midfielder Vieira has missed the last two games due to a knock sustained in the recent win at Burton, while an ankle injury has kept winger Dallas sidelined since the end of November.
Right-back Luke Ayling will be assessed after an ankle knock forced him out of the goalless draw against Nottingham Forest on New Year’s Day.
Head coach Thomas Christiansen is expected to rotate his squad, with the likes of Pierre-Michel Lasogga, Vurnon Anita, Hadi Sacko and Mateusz Klich pushing for recalls. Striker Caleb Ekuban (foot) is still out.