Mesut Ozil is sick for the fourth time in just over a year.
"Ozil's immune system is a bit in trouble at the moment and I don't know why. I hope we can improve that medically," said Arsene Wenger.
Ozil will miss Thursday's Europa League match against Ostersunds.
Wenger said Ozil was set to "work hard" on Thursday in order to be fit for Sunday's Carabao Cup final against Manchester City.
"He should be OK for Sunday," Wenger said.