The fullback was sold to the Zebrette for €15m and immediately loaned back for the season.
Duxbury said: "This allows us to protect the club in an era in which to balance the financial aspect and build a team that competes at the top of the Championship ( represent our main priorities.
"Speaking with the coach, then at the end of the season we will review our options, also depending on the division."
Kamara was Watford's Player of the Year last season.
