Marsch recently saw Mark Jackson depart his backroom team to take charge at League One strugglers MK Dons.
The American has moved to replace him and is bringing in his countryman Armas, who worked alongside Ralf Rangnick at Manchester United.
Armas is ready to come into the role at Leeds and is waiting for his visa to start work in the UK.
Marsch will hope to see Armas help him steer Leeds up the Premier League standings during the second half of the season and away from relegation trouble.
Leeds are only two points above the bottom three, though in a congested lower third of the league they sit 14th.
The Whites scored a morale-boosting win earlier this week when they progressed in the FA Cup, knocking Cardiff City out.
They are next in action on Sunday when they welcome eighth placed Brentford to Elland Road in the Premier League.
