Shanklin was originally expected to be out of action for three months but has conceded he will not play again this season and fears his career is in the balance.
"My knee is in pretty bad shape really and so I'm just going to see how I recover," said the 70-times capped Shanklin.
"I'm not going to play again this season. I'm going to make sure my knee is right and I'll take it from there. It's quite a serious injury I've got."
Shanklin's career has been blighted by knee problems and they resurfaced during a Magners League clash against Aironi in January.
The Cardiff Blues centre is also out of contract in May. Cardiff had indicated that they would not renew Shanklin's or fellow Lion Martyn Williams' contracts at the end of the season.
They are now believed to be working to keep three-tour Williams, but there has been no indication over securing Shanklin's future in Cardiff.
But Shanklin insists he is not worried about his contractual situation, only focusing on recovering from this latest injury.
"It's not something I'm really focusing on too much," said Shanklin.
"I mean my knee is pretty bad. I want to get that right. And if I get my knee right, I'll get a contract."
Shanklin toured New Zealand with the Lions in 2005 and was selected to tour South Africa in 2009 only for a shoulder injury to rule him out. Shanklin pulled on the Lions jersey on three occasions during the 2005 tour and scored one try.