The West Ham boss believes it is "crazy" that his colleague is being replaced at Vicarage Road after guiding the Hornets to Premier League safety following promotion.
West Ham coach Slaven Bilic has hit out at Watford's decision to replace Quique Sanchez Flores despite achieving Premier League survival.
The Hornets enjoyed a superb start to life back in the top flight after promotion from the Championship but a more difficult second half of the season has seen them fall to 13th place.
Goal understands ex-Napoli and Inter boss Walter Mazzarri will take over at Vicarage Road for the 2016-17 campaign, but Bilic labelled Flores' departure "sick.