Lundgren, who came on with 10 minutes to play to win his second cap for Sweden, went down in the warm-up on Monday ahead of GAIS' first game of the Allsvenskan season against Djurgarden, and played no part in the 1-0 home defeat.
The initial reports before kick-off at Gamla Ullevi were serious, as the 30-year-old was seen hobbling off the pitch, helped by two of GAIS' medical staff.
It has now been confirmed that the winger has ruptured his Achilles tendon, and will play no part in the World Cup, and could miss the entirety of the 2026 Allsvenskan campaign.
Lungren spoke to Swedish television after the match, still trying to process the injury and the news.
"I'm a bit in shock. I was just going to step back and take a small explosive step, and then I felt something happen," said the GAIS man.
Blagult boss Graham Potter is now tasked with finding an alternative to a player who featured where it mattered most for the Swedes; Lundgren was involved in the flurry of chances that eventually led to Viktor Gyokeres' winning goal.
"Of course you would have wished it had been a different occasion, to go there. When you have a lot to look forward to and have the chance to experience a World Cup, it would have been fun," Lundgren admitted.
