Xherdan Shaqiri has become the third Liverpool player to test positive for coronavirus over the past week.
The forward, who remained at Anfield despite having been linked to the likes of Sevilla and Lazio ahead of deadline day, travelled to link up with the Switzerland squad on Monday.
However, the Swiss Football Association on Tuesday revealed Shaqiri returned a positive test for COVID-19 and he will now undergo a period of self-isolation.
"Xherdan Shaqiri has tested positive for Covid-19. In consultation with the health authorities, he is in isolation," a statement read.
"The strict protective measures and requirements of UEFA for national teams in accordance with the 'UEFA Return to Play Protocol' have been strictly adhered to since the move began.
"Based on this, clarifications are currently underway with the local health authorities about how to proceed."
Shaqiri consequently misses Switzerland's friendly against Croatia and Nations League fixtures with Spain and Germany.
He is the third player in Liverpool's squad to test positive for the virus since last week, with Thiago Alcantara – who arrived from Bayern Munich in the transfer window – confirming he had contracted COVID-19 last Tuesday.
It was then revealed Sadio Mane had also tested positive last Friday, with Liverpool then going on to lose 7-2 at Aston Villa on Sunday.
All three players could still play against Everton in the Merseyside derby on October 17, though, as protocols require those with positive tests to go through a quarantine period of 10 days.