The former Fulham and Newcastle United striker is in his second season in the Saudi Pro League.
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Mitrovic struck 21 goals for Al-Hilal last season and made a return from hamstring injury against Al Feiha on the weekend, having been two months out. Mitrovic scored in the 2-0 win.
He later posted: "It has been a tough two and half months for me, had some problems with my hamstring, injury... after injury.
"I worked really hard to come back... I can help my teammates... happy with the goal."
However, the Serb is now in hospital with the club reporting the admission was made due to a 'rapid heartbeat caused by physical stress'.
Mitrovic has been instructed to take 'complete rest' after Sunday's hospital admission.