"Oscar spent the evening in hospital being assessed by doctors and will undergo further tests today," a statement said.
"Assistant coaches Ruben Martinez and Javier Pereira will take charge of Watford for the clash with the Seasiders at Bloomfield Road".
Garcia joined the club this month having ended his second spell at Maccabi Tel Aviv after less than three months.
He was previously in charge of the Barcelona youth team and Brighton.
Watford are in fourth place in English football's second tier after losing 1-0 at Charlton on Saturday.
(Writing by Steve Tongue, editing by Ed Osmond)
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