Phelan has made just the one change to the line-up that lost the final warm-up match 28-13 to Wales - with Gonzalo Tiesi named ahead of Marcelo Bosch at outside centre.
And Phelan admitted: "We think that for this game and this moment it is the best team we have.
"We know we have 30 players and all are in very good condition for playing, but... this starting XV is the best players we can put on the field."
The only possible surprise in the Pumas line-up comes in the front-row, with Juan Figallo retaining his place ahead of the experienced Martin Scelzo, who is appearing at his fourth World Cup.
However Phelan has backed the 23-year-old, who plays for Montpellier in the Top 14, to shine in Dunedin on Saturday.
"We know he is a young player but he is developing very well," Phelan said.
"He had a very good year in Montpellier and we think he is the best option for the right prop and also he can play left so Figallo is a very good option.
"We have Scelzo on the bench and that's also a right prop in good condition."
Argentina: 15 Martin Rodriguez, 14 Horacio Agulla, 13 Gonzalo Tiesi, 12 Santiago Fernandez, 11 Gonzalo Camacho, 10 Felipe Contepomi, 9 Nicolas Vergallo, 8 Juan Martin Fernandez Lobbe, 7 Juan Manuel Leguizamon, 6 Julio Farias Cabello, 5 Patricio Albacete, 4 Manuel Carizza, 3 Juan Figallo, 2 Mario Ledesma, 1 Rodrigo Roncero.Replacements: 16 Agustin Creevy, 17 Martin Scelzo, 18 Mariano Galarza, 19 Alejandro Campos, 20 Alfredo Lalanne, 21 Marcelo Bosch, 22 Juan Jose Imhoff
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