Reports in Spain have confirmed the Dutch international returned for preseason tests following the Netherlands elimination from the 2026 FIFA World Cup last month at the Round of 32 stage.
However, despite optimism over the former Ajax playmaker being OK after the tournament, medical tests on his return to Catalonia have uncovered a serious knee problem.
Confirmation on how bad the injury is, and De Jong's expected absence period should become clear in the coming weeks, with swelling in the area currently preventing further checks from taking place.
As per the latest from Marca, Barcelona are angry with the Netherlands' lack of care over their star midfielder, who looks certain to at least miss the start of the 2026/27 season.
The situation could have a knock-on effect for Barcelona's transfer plans, but Marc Casado is still expected to leave the LaLiga champions, amid rumoured links with other European sides and the Saudi Pro League.
Casado has slipped down the midfield pecking order at Barca and Hansi Flick is open to him leaving despite the news on De Jong.
Barcelona want to offload the Spain international, to try to bring in around €30M in revenue, as Flick is confident Marc Bernal and Gavi can cover the gap created by his departure.