The Borussia Dortmund star has not travelled with the rest of the squad to Munich, where they are due to play Bayern at the Allianz Arena.
In a tweet, the club said both Sancho and goalkeeper Roman Burki were suffering from the illness, but added both had tested negative for coronavirus.
With less than a week until the transfer window shuts, it looks increasingly likely that Sancho will remain at Dortmund this season, with club captain Marco Reus suggesting as much on Tuesday.
The 20-year-old has been Manchester United’s no.1 transfer target all summer but no deal have yet been agreed between the clubs and the Bundesliga side insisting he is not for sale after United failed to meet their August 10 deadline.
Sancho has started each of Dortmund’s Bundesliga games so far this season and scored his first goal of the new campaign in the German Cup win over MSV Duisburg.