Montoya had to leave the pitch after suffering a head injury in the 42-12 defeat to Ireland in Dublin last Friday.
The Pumas suffered another blow on Friday when Juan Cruz Mallia sprained his right knee, leaving them short at fullback with Santiago Carreras and Benjamin Elizalde already injured, the Argentine Rugby Union said in a statement.
The uncapped 27-year-old Tobias Wade joins the squad as a replacement for Mallia, while Bautista Bernasconi and Facundo Bosch are added to the squad as cover for Montoya. Bosch won his last cap for Argentina five years ago.
Uncapped prop Matias Medrano has replaced Rodrigo Martinez in the squad, while number eight Facundo Isa has returned to the camp after being released to play for Pau in the French Top 14 last weekend.
Argentina are training this week in Parma, some 200 kilometres from Genoa, where they will take on Italy.
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