Granit Xhaka has tested positive for Covid-19, the Swiss federation announced on Wednesday.
The Arsenal midfielder was left out of the squad to face Greece on Wednesday after showing symptoms earlier in the day.
A PCR test produced a positive result, ruling Xhaka out, while a member of Switzerland's backroom staff has also gone into quarantine.
What was said?
"Xhaka was diagnosed with symptoms on the morning of the game and was immediately isolated in his room," the federation said in a statement.
"A quick test turned out negative, the subsequent PCR test, carried out to be on the safe side, produced a positive result in the evening. Xhaka stayed out of touch with his team-mates all day.
"He will do another PCR test on Thursday.
"The canton doctor of Baselland ordered no further measures for the team because the other players in the senior national team are all either vaccinated or have recovered [from Covid-19]. A member of the staff had to go into quarantine."
Up next for Xhaka
In addition to missing out on the clash with Greece, Xhaka looks like to miss out on a match against Italy on Sunday in Basel.
Following the conclusion of the international break, Xhaka is set to serve a suspension after being sent off in Arsenal's 5-0 loss to Manchester City over the weekend.