The game at Vicarage Road has been moved from July 30 to Monday, August 1 for television coverage, with United’s first home game at Bramall Lane coming the following Saturday when they welcome Millwall.
The Blades are at home on Boxing Day, to Coventry City, and finish the season away at Birmingham City on May 6.
All the fixtures are subject to change for television scheduling.
United don’t have to wait long for another reunion with former boss Chris Wilder, travelling to Middlesbrough on August 13, while derbies with local rivals Rotherham United are on September 10 (home) and February 4 (away).
United’s last game before the World Cup break is away at Cardiff City on November 12. They return to action on December 10, at home to last season’s beaten play-off finalists Huddersfield Town.