Jonathan Sexton and Rhys Priestland were the leading candidates to tour Australia in the summer but have been sidelined by injury.
Sexton returns to the Leinster side, for the first time since January, to face Zebre in the PRO12.
The Ireland star endured an injury-ravaged Six Nations. He suffered a hamstring strain during the tournament and then damaged a tendon in his foot ahead of Ireland's Championship finale against Ireland.
But he is one of 13 changes made by Joe Schmit ahead of Leinster's Amlin Challenge Cup semi-final with Biarritz Olympique, with Brian O'Driscoll and Isa Nacewa remaining in the starting line-up.
Meanwhile Priestland will also make his comeback in the Scarlets clash with Cardiff Blues.
The 26-year-old ruptured his Achilles in December and missed the entire Six Nations following surgery.
It was expected that Priestland would miss the remainder of the season, virtually ending his Lions hopes.
His slim hopes have been kept alive by his inclusion on the bench for the Scarlets crucial Welsh derby but he has down played his chances of touring.
"I've passed all the tests and feeling good after the rest so now it's just about getting back on the field and enjoying my rugby again," said Priestland.
"All I am thinking about right now is helping the Scarlets reach the play-offs. We have missed out in recent years and it would mean a lot to be part of that.
"Whatever happens after that is out of my hands."
Warren Gatland names his Lions squad on April 30, and has indicated he will select just two specialist fly-halves.
Owen Farrell, Toby Flood, Jonny Wilkinson, Dan Biggar and Ian Madigan are the other likely candidates.