The highly-rated defender is heading to Vicarage Road until January 2 to play under Blues' legend Gianfranco Zola, after completing a loan move to The Hornets
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The 17-year-old was captain of the FA Youth Cup winning side last season, and also skippered England at U-17 level last term, as well as regularly wearing the armband for the Blues' reserves.
The central defender, who can also play in midfield, signed a two-and-a-half year deal with the west London club in January but has never made a senior appearance, though he has been an unused substitute.
Therefore, Roberto Di Matteo has decided to allow the Englishman, who put in an impressive performance for the club in their first ever NextGen clash against CSKA Moscow at Cobham on Tuesday, to gain some experience away from the club.
The former Chelsea schoolboy will now work under club legend Zola at Watford, with the Italian taking charge of the Championship outfit in July after the Pozzo family's purchase of the team.
Hopes are high for Chalobah at Stamford Bridge, with the teen making his England Under-17 debut at just 14, before he became a European champion at the same level in 2010.