The England fly-half will have his first taste of international rugby since kicking the winning points in the World Cup final 18 months ago when he lines up against the Pumas at the Millennium Stadium.
"It is exciting to have players of this quality to work with and talk to and that reflects what the Lions is all about," said McGeechan.
"You see world-class players at a distance - Brian O'Driscoll is another - and to get to coach them and work with them on the same field excites you.
"I have been very impressed with Jonny. It is good to see him back. He is fully fit, he is keen.
"With his injuries and coming in to the international environment it is like a new start. That is an exciting concept from all our points of view."