Winger Duffy has missed the last two games due to a knee problem, while striker Lavery was withdrawn from the midweek win at Swindon after complaining of a tight hamstring.
Manager Chris Wilder has no new injury or suspension problems as the Blades bid to extend their eight-point lead at the top of the table.
Striker Leon Clarke is back in training after being sidelined since mid-January with an ankle problem.
Jason Pearce could return for Charlton after playing for the club's Under-23s last weekend.
He had been missing with a groin problem suffered in December, but fellow defender Lewis Page is out because of a hamstring injury.
Jake Forster-Caskey and Ezri Konsa are again in contention to start, having returned as substitutes during Tuesday's 1-1 home draw with Bradford.
Josh Magennis, owing to a hamstring injury, is again expected to be unavailable.