The 24-year-old went over on his knee during Northampton’s 21-19 Premiership semi-final defeat to Saracens last weekend and now faces a battle to be fit for the start of next season.
Hartley’s injury comes as a massive blow to Johnson after enjoying a fine season. Not only was he promoted to Saints captain, but Hartley also established himself as an England regular in the No2 shirt, starting in the last seven Tests.
Brive’s veteran front-rower Steve Thompson and Bath’s Lee Mears are the men most likely to replace the New Zealand-born Hartley in the 44-man squad bound for Australia next week, joining fellow Saint George Chuter and Wasps’ Rob Webber.
Meanwhile, the England Saxons have also suffered a blow after Wasps flanker Tom Rees pulled out of the squad, failing to recover fully from a shoulder injury.
Leeds’ Kearnan Myall comes in for Rees, while Newcastle’s James Hudson replaces Bath second row Stuart Hooper, who has a toe injury. London Irish hooker David Paice also misses out with a quad strain. His replacement is yet to be named.