The Spaniard was admitted after suffering chest pains last Monday, meaning he was absent from the 1-0 Championship victory at Blackpool the following day, as well as the 1-1 draw at home to Bournemouth on Saturday.
The 41-year-old took over from Giuseppe Sannino earlier this month and lost his first match in charge, 1-0 at Charlton Athletic.
Watford refused to place a timeframe on his return on Saturday, but on Monday announced that he left hospital over the weekend.
"The club is pleased to confirm that head coach Oscar Garcia left hospital yesterday [Sunday]," a brief statement read.
"At this stage, the Hornets expect Oscar to hopefully return to work later on this week."
Watford next face Blackburn Rovers at Ewood Park on Saturday.
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