Former Newcastle United and Fulham striker Aleksandar Mitrovic missed training for his club Al Hilal on Sunday with an irregular heart beat, the Saudi side said.
The 30-year-old Serbian international scored in the Saudi Pro League on Friday having been out of action since the start of the year with injury.
He was in line to play in Al Hilal's home second leg against Uzbekistan's Pakhtakor in the Asian Champions League last 16 on Tuesday.
He is now a doubt after Al Hilal said in a statement: "The medical staff has decided to rest Aleksandar Mitrovic from Sunday's training due to feeling fatigue and arrhythmias.
"He will be observed accurately by specialists, as his case will be monitored constantly."
Al Hilal, record four-time winners of Asia's top club competition, trail 1-0 after the first leg in Uzbekistan.
Mitrovic joined Riyadh-based Al Hilal from Fulham in August 2023 for a reported fee of about 50 million euros.
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