Previously Mackay was a player agent for London-based Stellar Group.
United and Nashville SC officials said they did not want to comment.
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Mackay will become the most powerful figure on United’s technical side, tasked with setting the team culture and leading the search for a head coach following Wayne Rooney’s departure in October.
Mackay will, in essence, replace Dave Kasper, who, with multiple titles over almost 22 years, has supervised most roster moves. Kasper is currently United’s sporting director and president of soccer operations.