Chris Wilder believes United will face a tough challenge at Bramall Lane on Saturday.
And the Blades boss says nothing can be taken for granted against Reading, despite United's position in the Championship table.
Speaking about Jaap Stam's Royals, Wilder remarked: "Like everyone in this division they have major strengthens and we've got to be aware of those.
"We're talking here about a side that last season came within a penalty-kick of winning promotion to the Premier League.
"They've spent a bit in the summer and are an established Championship club, so we will have to overcome several difficulties to beat them."
And the Blades boss is demanding more from his side, adding: "We've shown plenty this season and we've been in sparkling form at times, but we know we will have to be at our best again to get a result, if we're not we leave ourselves vulnerable.
"Reading got a fantastic win at Leeds, so their confidence will be high and they will be looking for another away win in Yorkshire."