A Champions League-winner with Porto, the striker will join manager Erik ten Hag's backroom team.
It will be a dream move for the 44-year-old who is a lifelong fan of the Premier League giants.
McCarthy had an impressive playing career which saw him land Europe's elite trophy with Porto in 2004.
He also played for Ajax, Celta Vigo, Blackburn Rovers and West Ham before he ended his career his his homeland with South African side Orlando Pirates.
Ten Hag brought Steve McClaren and Mitchell van der Gaag with him upon his arrival at Old Trafford.
Now, though, the Dutch boss has made another addition with McCarthy reportedly tasked with, amongst other roles, helping forwards with their finishing and positioning.
He previously managed Cape Town City and Amazulu.
In an interview with FourFourTwo back in 2017, McCarthy revealed he was admonished back in 2004 for his reaction to helping Porto beat the Red Devils in the Champions League.
He netted twice in the 2-1 victory in Portugal before a 1-1 draw sent them into the quarter-finals of the competition.
"I support Man United and always wanted to play for them," he said. "I loved players like Mark Hughes, Andy Cole and Ryan Giggs.
"So I was heartbroken to score twice. My dream was to score once at Old Trafford, not to knock United out of the Champions League. I got hammered after the game by Jose Mourinho because I wasn't very happy. He told me that if I didn't cheer up then I'd never play in his team again."