Hakim Ziyech has a "serious" injury, says manager Thomas Tuchel, after the Moroccan winger appeared to suffer a shoulder problem in Wednesday's Uefa Super Cup victory over Villarreal.
Ziyech opened the scoring but hobbled off before half-time following an innocuous looking challenge with ex-Tottenham defender Juan Foyth.
He sat in the stands for the second half with his arm heavily strapped up.
"If you take a player out during the first half it's serious," said Tuchel.
Ziyech looked to be in quite a bit of discomfort and was given an inhaler-type device to help cope with the pain after suffering the injury on 39 minutes, just 12 minutes after netting Chelsea's goal.
The match in Belfast ended 1-1 but the Blues were victorious 6-5 on penalties after substitute Kepa Arrizabalaga saved two penalties.
Chelsea begin their Premier League campaign against Crystal Palace on Saturday (kick-off 15:00 BST).
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