The 24-year-old was forced off during Newcastle's 2-2 draw with West Ham in the Premier League on Sunday, with the player on target twice for his side.
Isak then reported for international duty with Sweden ahead of their upcoming friendly with Moldova and Euro 2024 qualifier against Belgium.
Though it has no been revealed the former Real Sociedad star will play no part in those matches, after medical staff carried out necessary tests.
"Alexander Isak leaves the men's national team's European Championship qualifying squad," a statement read.
"The Newcastle striker came to the gathering with an injury report from Sunday's match against West Ham and underwent an examination by the Swedish national team's medical team on Monday.
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"After that, it is clear that Isak is not available for games next week and will miss the European Championship qualifier against Belgium next Monday."
It is a huge blow for Newcastle fans who are now concerned following his setback.
Isak has been in stellar form for the Magpies, netting six times in seven Premier League outings so far, including his double against the Hammers.
The 22-year-old joined the club for a record £63million in 2022, and is already a fan favourite on Tyneside.
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He could miss Newcastle's next clash after the international break against Crystal Palace on October 21 at St James' Park.
It means Callum Wilson looks set to be given the nod, having replaced the Swede against West Ham at the weekend.