Mitchell has done well as a 'head of recruitment' in his career thus far and isn't doing very well at all as the head of football operations at Monaco, hence a lot of their fans are not happy with him.
Mitchell had no experience of running/overseeing multiple departments on the football side at a high level for a period of time before he was given the Monaco gig. The likes of Murtough (United), Ashworth(Brighton), Webber (Norwich), Crocker(Soton) McDermott (Spurs), all had one thing in common before they became the head of football operations at their respective clubs/associations, and that was that they had a lot of experience being the ones who were directors at academy level. Which meant they had to make hiring/firing calls and oversee the development of multiple departments beneath the first team, which was controlled by the first team manager.
Mitchell came into prominence when he was working under the Soton DoF, Les Reed. And it was Les Reed who brought Mitchell and Pochettino to Southampton. Mitchell was the Head of recruitment at Southampton and reported to the DoF Les Reed. At Spurs, Mitchell was also the head of recruitment and reported to either Levy or Pochettino in a manager led approach to the first team. And even when Mitchell joined RB Leipzig, he was the Head of recruitment at the club reporting to their DoF, Ralf Rangnick and his assistant. And when Rangnick stepped down from his role as the DoF, Leipzig didn't appoint Mitchell to replace Rangnick but rather opted for Markus Krösche who replaced Rangnick as the DoF at the club.
And what role is available at United that Mitchell has done well in the past in? That's right, it's the role of Head of recruitment.
If Mitchell does leave Monaco as its been reported by some media outlets then I can't see why he'd get the role of the Sporting director for the reasons stated above. But I can see him being chosen for the role of head of recruitment or in a deputy DoF role. Because even Rangnick had his former player from Hoffenheim, Per Nillson, to assist him at the Redbull clubs as the assistant Sporting director. And Nillson had only just retired from playing at Hoffenheim when he was given the role at 34 years of age in 2017, without any experience.