Everton boss Marco Silva pledged to seek answers from referee Lee Probert after Kurt Zouma was sent off after full-time of the 1-0 loss at Watford.
The Everton players fumed as Andre Gray's 65th-minute goal proved decisive, insisting the corner that led to it should not have been awarded.
Zouma continued to voice his disquiet following the conclusion at Vicarage Road and was shown two yellow cards.
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"I will try and speak with the referee now if he stays here because I spoke with Kurt at the end of the match," Silva told a post-match news conference after Everton's eighth defeat in 11 Premier League matches was lapped up by fans of his former club Watford.