The team said in a statement that injury occurred Thursday after a collision in the corner during two-on-two work at rookie camp practice. X-rays confirmed the fracture on Monday.
The injury will likely keep the 19-year-old out of the entire pre-season and part of the regular season.
The Lightning start the 2014-15 season Oct. 9 on the road against Florida.
Drouin, from Ste-Agathe, Que., was selected third overall by the Lightning in the 2013 NHL draft.
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