Chris Wilder believes Saturday's Sky Bet League One encounter with MK Dons could be one of the most difficult games of the season.
The Dons sit 19th in the table but parted company with manager Karl Robinson last week after a tough start to the campaign following their relegation from the Championship last term.
"Sometimes a bounce of a ball, a bit of bad luck or injuries to key players can change wins into draws or draws into defeats and maybe that's what has happened with MK Dons this season," said Wilder.
"I'm surprised they are not up around the top, they lost a couple of players but they have brought a couple in. I thought, with the settled group they had, they would go very well."
Wilder added: "Regardless of their issues and their league position we have to look at them as a team and the threats they have. I think it will be one of the toughest games we have at Bramall Lane this season, we have to approach it in the right way.
"They could be flat after losing the manager or they could raise their game, you just don't know. We are in the dark a bit, but we will respect what they have."
Sheffield-born Richie Barker is expected to be in temporary charge when the Dons arrive at the Lane on Saturday.