The news comes as a hammer blow to the flanker, who had worked so hard to return to competitive action last season after a career-threatening neck injury.
And Leicester Tigers director of rugby Richard Cockerill admitted he could scarcely believe the run of bad luck his flanker is having.
"His season's finished," said the Tigers boss.
"It's horrendous bad luck for Tom because it is only last year that he came back from a very bad neck injury and did so well, not only for England but the Lions as well."
Barritt meanwhile, has had to undergo surgery on his ankle ligaments after re-injuring an old problem in Saracens' season opener against London Irish.
Barritt was called up from a holiday to join the Lions touring party as injury cover and went on to appear in two clashes, against the Brumbies and the Melbourne Rebels.
And Sarries head coach Mark McCall is confident that, unlike his England teammate Croft, Barritt will not miss the entire domestic season and is already looking forward to his return.
"Brad is a tremendously important player for us and England. He is on the road to recovery and we look forward to his return later this season," said McCall.
"He will be missed through the autumn, although we have talented players ready to step forward."