Armas was fired by Toronto in July after less than six months in charge. During his time at the helm, Toronto suffered a 7-1 defeat to D.C. United – the worst margin in the club's history.
Nevertheless, Armas will look to redeem himself at Manchester United after being recruited as the first new member of Rangnick's backroom staff.
One big advantage to the move is that Rangnick and Armas already know each other, having previously working together at the New York Red Bulls.
Rangnick was the head of sport and development for Red Bull's clubs in New York and Brazil while Armas was appointed as the head coach of the American team.
ESPN says Armas was given his work permit on Monday morning after initially being denied for not having a UEFA Pro License.
Having won his appeal, Armas will now line-up alongside Rangnick in the Manchester United dugout as the German's assistant manager.
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