Blades boss Gary Speed had lined up a loan switch for De Laet to help bolster their defensive options.
The Belgian international is highly-rated at Old Trafford and he has made six first-team appearances since arriving from Stoke in January 2009.
De Laet will go straight into the Blades squad for Saturday's clash at Leeds and Speed believes he will prove a valuable addition to the squad.
"We need versatile cover in case of further disruption and Ritchie will go straight into our squad at Leeds on Saturday.
"I have seen him play loads of times, he's a good, solid player and I'm very pleased to get him here."