The Red Devils are building towards their Premier League opener against Leeds United on August 14, while Derby will start their Championship season at home to Huddersfield Town on August 7.
There is no getting away from the fact that 2020-21 proved to be a difficult campaign for Derby, who collected just 44 points from their 46 Championship matches to finish down in 21st spot, just a point above the relegation zone.
The Rams had still been threatened with relegation to League One, as the EFL considered an appeal against a £100,000 fine issued for financial irregularities, but Rooney's side will play in the Championship next term.
Derby are currently operating under a transfer embargo, though, so can only sign free agents and loan players, which has placed them in a difficult position ahead of 2021-22, which starts with a home game against Huddersfield on August 7.
Rooney's team have four confirmed friendlies in the diary over the next couple of weeks, with clashes against Salford City, Real Betis and Notts County to come.
The Rams will hope to have boosted their squad with some free agents and loan signings in the coming weeks, and former Man United youngster Ravel Morrison is one of the players currently undergoing a trial period.
Man United, meanwhile, have not been in action since losing the Europa League final on penalties to Villarreal at the end of May; there will be Champions League football next term, though, courtesy of a second-place finish in the Premier League, which has to go down as a success.
The 20-time English champions are on the verge of confirming a deal for Jadon Sancho, while Raphael Varane could arrive from Real Madrid, with Ole Gunnar Solskjaer looking to assemble a squad capable of mounting a serious challenge for next season's Premier League title.
The Red Devils have five pre-season matches in the diary, following this clash with fixtures against Queens Park Rangers, Brentford and Preston North End before the end of the month.
Solskjaer's team will then take on Everton in their final pre-season fixture on August 7 ahead of their 2021-22 Premier League opener at home to Marcelo Bielsa's Leeds on August 14.
Man United's start to the new season is relatively kind on paper, and it will be fascinating to see whether the club are capable of launching a title challenge; for now, though, the players will be fully focused on getting up to speed in the coming weeks.
However, Morrison, Sone Aluko, Sam Baldock, Richard Stearman, Phil Jagielka and Ryan Allsop are currently part of the team's training camp in Surrey, and the trialists could be involved against Man United.
Defenders Andre Wisdom and Curtis Davies are also still training with the Rams despite seeing their contracts expire at the end of June, and it will be fascinating to see what side is sent onto the field from the start.
As for Man United, the players involved in Euro 2020 and the Copa America will not be available for selection, while Ivory Coast duo Eric Bailly and Amad Diallo have been selected for the Olympics and will therefore not feature.
Recent arrival Tom Heaton could play between the sticks, while Nemanja Matic, Aaron Wan-Bissaka, Jesse Lingard and Juan Mata will be among the senior players looking to build their fitness in the team's pre-season opener.
Ethan Laird, Anthony Elanga, Hannibal Mejbri, Mason Greenwood, Shola Shoretire, Facundo Pellistri and James Garner should be involved for the Red Devils, as Solskjaer will hand opportunities to a number of younger players.
Andreas Pereira is also back at Old Trafford following a loan spell at Lazio, and it will be interesting to see whether the midfielder is in the plans of Solskjaer for the 2021-22 campaign.
Marcus Rashford would not have been available even if he had not featured for England at Euro 2020, though, as the attacker is preparing to undergo a shoulder operation at the end of the month.
Head To Head
Man United lead the overall head-to-head record between the two sides, having recorded 47 wins to Derby's 32, while there have also been 29 draws throughout history.
The pair have not met in the league since March 2008, though, with Man United running out 1-0 winners at Pride Park courtesy of a late winner from Cristiano Ronaldo.
We say: Derby County 1-2 Manchester United
It is difficult to know exactly what to expect from this fixture, as there are likely to be wholesale changes from both managers throughout the contest. The result is not too important for either side at this stage of their preparations, but we are expecting Man United's quality to help them pick up the win.
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