Arsene Wenger said Mesut Ozil's immune system is "a bit in trouble" after the German playmaker was bedridden with an illness again this week.
Ozil has been ruled out of Thursday's Europa League game against Ostersunds after he spent Monday and Tuesday in bed, Wenger said -- although he may have been rested anyway given Arsenal's 3-0 lead from the first leg.
It's the fourth time in little over a year that Ozil has been sick. He missed four games with illness in the 2016-17 season, as well as matches against Huddersfield Town and Burnley in November.
Asked why the German gets sick so often, Wenger said: "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."
Ozil played the full 90 minutes in freezing temperatures in Ostersund last week, and Wenger said he trained as normal this past weekend before getting sick on Monday.
Ozil was back in full training on Wednesday, and Wenger said he 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.