Former Newcastle and Fulham striker Aleksandar Mitrovic was omitted from Al Hilal's matchday squad for Tuesday's AFC Champions League Elite tie against Pakhtakor, after being found days previously to have an irregular heart beat.
The 30-year-old Serbian international marked his return to the team on Friday by scoring in the Saudi Pro League – his first appearance since the start of the year following a hamstring injury which caused him to miss 13 matches.
However, on Monday 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."
Mitrovic was not included in Tuesday's second leg against Uzbekistan's Pakhtakor in the AFC Champions League Elite last 16 in Riyadh. Brazilian forward Marcos Leonardo will start as striker in his absence.
Al Hilal, record four-time winners of Asia's top club competition, trail 1-0 after last week's first leg in Uzbekistan.
Mitrovic joined Al Hilal from Fulham in August 2023 for a reported fee of about €50 million.
He played a key role in the Riyadh-based club winning last season's Saudi Pro League by scoring 28 goals in 28 games.
Mitrovic has added another 13 league goals in 14 matches this season, while he has five goals in six appearances in the rebranded Asian Champions League.