The former Australia international utility-back was due to join the Saints on a three-year deal earlier this month after seven seasons with Super 15 side Western Force.
However, Northampton pulled the plug on the 28-year-old's move to Franklin's Gardens on medical advice. He has since joined their Aviva Premiership rivals Sale.
Director of rugby Jim Mallinder said at the time: "We obviously monitored Cameron's condition between signing his contract and his scheduled arrival and, after great consideration, felt that on balance we needed more certainty about his ability to handle the tough demands of our long season for the duration of his three-year contract,"
Shepherd's agent issued a statement on Thursday claiming he was "appalled at the behaviour of Northampton in respect of the manner in which his client and family were treated".
Nick Fordham said: "Expert medical reports from leading independent orthopaedic surgeons and Cameron's former club doctors at the Western Force have concluded that there is nothing wrong with Cameron's ankle."
Shepherd has been troubled by groin and wrist injuries and played just two games for the Force this year.
He made his Super rugby debut with the New South Wales Waratahs in 2004 and played nine tests for Australia between 2006 and 2010.