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The Dutch striker has hit some strong form in recent days, following a brace at Carrow Road last weekend and an assist in Tuesday's win over Lincoln City.
But, speaking on Friday, head coach Alessio Dionisi cast doubt over his involvement against Middlesbrough, after saying the 25-year-old was "more or less" okay after being seen with strapping on his knee after the Lincoln win.
The Italian said a late decision would be made on his potential inclusion, and it is understood that he failed that late fitness test due to a leg issue.
With Hector Kyprianou already sidelined due to suspension, Boadu's absence comes as a huge blow for Watford ahead of their toughest game, on paper, in recent weeks.
Middlesbrough are flying high at the top of the Championship, and despite falling to a 2-0 defeat in Wrexham on Wednesday, they are heavy favourites for the test.
Coming into the game off the back of a win and ahead of the international break, a win would go a long way for Watford and Dionisi.