The 29-year-old New Zealander twice suffered a dislocation after coming off the bench in the Guinness Premiership champions' 20-19 win over Gloucester on January 6.
A club spokesman said: "Lilo' injured his shoulder against Gloucester in a collision. The shoulder was dislocated twice in the game but he bravely played on.
"After consulting a specialist, it has been decided that he will undergo a full shoulder reconstruction and will be out of action for 16-18 weeks."
Meanwhile, the club has been confirmed that Nathan Bonner-Evans will undergo a full knee reconstruction within the next two weeks.