John Muldoon tore his medial knee ligament in Saturday's 53-0 win for the Ireland XV over Fiji in Limerick.
He is likely to be out of action for up to three months, ruling him out of Connacht's back-to-back Heineken Cup clashes with Biarritz next month.
Ruddock, son of former Wales boss Mike, is a straight replacement for Muldoon.
Fly-half Jonathan Sexton should be fit for the Pumas clash despite suffering a groin injury in the warm-up for the Fiji match, and had to skip the game.
Coach Declan Kidney said: "During the warm-up he got a bit of a tight groin so we felt it was prudent to pull him out."
The Ireland team and replacements for the crunch clash with Argentina will be named on Thursday afternoon.
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.