The 29-year-old suffered the injury during Saturday's 22-16 victory over Saracens in the Guinness Premiership.
Vainikolo underwent surgery at Cromwell Hospital in London on Monday and now faces around seven months on the sidelines.
And with the crunch meeting with Cardiff Blues less than three weeks away, Gloucester will be forced to rethink their strategy now that the try-scoring wing is absent.
"It will naturally leave a very significant gap and with the current injuries we have in the squad at the moment this could not have come at a worse time," admitted Gloucester head coach Dean Ryan
"Les is a massive part of our squad, not just for his on-field contributions but because his personality is such an integral and vital part of the group.
"Les has undergone surgery and when it is appropriate he will begin his rehabilitation process but we appreciate it will be a long road and he will have all the support he needs from everyone associated with the club."
Vainikolo - nicknamed the volcano because of his explosive power -