The Premier League club said Sarr was injured after Senegal accused Watford of refusing to release him for the tournament, which starts on 9 January.
Senegal confirmed that the 23-year-old left London for Cameroon on Tuesday.
Since November, Sarr has been out with knee ligament damage, and Watford invited his nation to send their own doctor to assess his injury.
FIFA rules state that any player called up for international duty must be released, even if injured, so the country's own medical staff can assess them.
Senegal's first Group B match is against Zimbabwe on Monday. Read Full Story
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