Dean Henderson will be ineligible to face his parent club at Bramall Lane on Sunday afternoon due to Premier League loan rules.
“Simon has been fantastic since he's came into the football club," said Wilder.
"He was massive for us in the League One season. He's been patient but he'll be looking forward to Sunday."
Wilder was also asked whether United, who have struggled under Ole Gunnar Solskjaer for much of this season, have lost their 'fear factor'.
But he dismissed the notion that the Blades were favourites to win when the sides meet on Sunday afternoon, adding: "I still think it's Manchester United, teams go through different periods."