Genia has taken over Gregan's mantle as the best scrum half in the world since bursting onto the scene with Queensland Reds and has returned this season "better than ever" since undergoing knee reconstruction.
"I think Will is getting better and better. How well he has bounced back from his knee reconstruction is amazing and he has gone past where he was before the injury, which says a lot about him," said HSBC Ambassador, Gregan.
"He has a huge impact on any game and he is always heavily involved. If you can provide him with the ball he is one of the best in the business in making you pay. He keeps people on their toes at the edge of the rucks, his longer passing game is fantastic and he gives players time and space.
"Beating the Lions is right up there - it is a once in a career opportunity. If Will can help Australia win this series he will go down in Wallaby history."
Gregan expects Lions head coach Warren Gatland to stick with Mike Phillips as his first-choice scrum half and then introduce England's Ben Youngs off the bench in the second half.
"The Lions have really good options at No 9 and they are all playing well. They all have good running games and they all pass well, but experience is going to count. Mike Phillips is hungry, has that big match mentality and plays at his best in the big matches," added Gregan.
"Then there is a good change of style in bringing on Ben Youngs. There is a wealth of riches there, but I think Mike will get the nod through his experience."
Gregan's only worry as the series gets set to start in Brisbane on Saturday night is that the Wallabies haven't played a game since they beat Wales with a last gasp try from Kurtley Beale in December. The Lions, meanwhile, will have had the advantage of playing six warm-up matches.
"The Wallabies will go into the first Test undercooked. They will be highly motivated to do well and fresh of mind, but if you make mistakes in a Lions series there is nowhere to hide - it is very unforgiving," said Gregan..
"If they can get the first Test over the line it will be huge. This is a massive series and it is all about momentum. There wasn't much between the Springboks ansd the Lions in 2009 and this won't be any different.
"The intensity of the first period in Brisbane will be incredible. This Lions are highly skilled and they can hurt you from set-piece and with a beautiful passing game and those first 20-30 minutes at Suncorp Stadium will lay out the platform for the series."