Both men were proteges of Erik ten Hag during the Red Devils manager's successful Ajax stint, and the Dutchman is determined to reunite with the pair at Old Trafford this summer.
Man United's pursuit of De Ligt was put on the back burner while the club concluded a deal to sign Leny Yoro from Lille, but the teenager will miss the first three months of the season with a foot injury.
As the likes of Everton's Jarrad Branthwaite and Benfica's Antonio Silva will prove too expensive, Man United quickly identified De Ligt as another top defensive target and will now go on the charm offensive following Yoro's injury.
Meanwhile, Mazraoui is being targeted as a replacement for Aaron Wan-Bissaka, who is now in the last 12 months of his contract and is said to have reached a verbal agreement with West Ham United, who are now working on a fee with Man United.
Bayern holding out for 'higher fee' for De Ligt, Mazraoui
According to The Athletic, Man United sought to kill two birds with one stone by submitting a joint bid for De Ligt and Mazraoui, which was sent to Bayern's offices on Thursday.
Both the Red Devils and the agents of the defenders were supposedly optimistic that the unspecified amount would meet Bayern's demands, as neither man is in Vincent Kompany's plans for the 2024-25 season.
However, the German giants are understood to have knocked back Man United's approach, and the 20-time English champions have been told to fork out a higher fee as talks rumble on.
Bayern's rejection of Man United's offer has allegedly left De Ligt and Mazraoui's agent - Rafaela Pimenta - bewildered, and their camps approached the club to ask if their clients were no longer deemed surplus to requirements.
Bayern issued the same response, but they are so far not showing a willingness to compromise on a fee, which they supposedly promised both men that they would do to help engineer their desired exits.
There are not expected to be any roadblocks on the players' side, as both have reportedly agreed personal terms with Man United over five-year deals with the option of an additional 12 months.
How have De Ligt, Mazraoui performed at Bayern?
After shining during Ajax's run to the 2018-19 Champions League semi-finals, De Ligt spent three years at Juventus before joining Bayern Munich in 2022, and he has five goals and two assists to show from 73 appearances.
De Ligt was included in the Netherlands' Euro 2024 squad, but he was an unused substitute throughout the tournament, as Ronald Koeman preferred Stefan de Vrij and Virgil van Dijk at the back.
Meanwhile, Mazraoui arrived at the Allianz Arena in the same summer and has amassed one goal and eight assists from 55 games, missing several matches through injury and health problems.