Luka Modric, Marcelo, Marco Asensio, Gareth Bale, Rodrygo and Andriy Lunin -- as well as assistant coach Davide Ancelotti -- all tested positive last week. Modric has subsequently tested negative, but remains unavailable as he continues to suffer from symptoms.
Neither Alaba nor Isco had trained on Tuesday as they awaited the results of PCR tests. They will both now be required to isolate at home for ten days.
All eight players will miss Madrid's match in Bilbao on Wednesday, as will the injured Dani Carvajal and Dani Ceballos and the suspended Casemiro. As a result, manager Carlo Ancelotti was forced to name a matchday squad on Tuesday containing just 14 first-teamers alongside seven youth players.
Real Madrid are currently top of LaLiga, although a ten-game winning run in all competitions was ended by a 0-0 draw with Cadiz on Sunday. A win against Athletic would guarantee that Madrid end the year at least six points clear at the top of the table, with second-placed Sevilla hosting Barcelona on Tuesday night.
Madrid then won't be back in action until Jan. 2, after LaLiga's Christmas break.
Speaking in a news conference on Tuesday before the latest positive test results were announced, Ancelotti said he was hopeful that the outbreak would be under control by then.
"We have seven days holiday," he said. "Let's hope they're all ready to train when we return."