The Netherlands international returned from the World Cup carrying the injury after playing through the tournament with a heavily strapped knee. The Dutch side were knocked out in the round of 32 after losing to Morocco on penalties.
De Jong revealed on Instagram that the damage was more severe than first thought but was relieved surgery will not be required.
"The injury was more serious than initially determined. Thankfully, at this stage there is no need for surgery, and I am now fully focused on my recovery," he said.
Barcelona also confirmed the extent of the injury in a medical update.
"Tests carried out on first-team player Frenkie de Jong have confirmed a tear of the medial collateral ligament (MCL) in his right knee.
"It has been agreed that the player will continue with conservative treatment under the supervision of the club's medical staff. His progress will be monitored over the coming weeks."
The 29-year-old will now begin his rehabilitation under Barcelona's medical team as the Spanish champions prepare for the new season.
Barcelona begin their bid for a third consecutive La Liga title away to Elche on August 23, with De Jong now set to miss the opening stages of the campaign while he recovers.
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Real Madrid are increasingly confident they can sign Manchester City midfielder Rodri before the transfer window closes, with the Spain international emerging as one of the club's top priorities.
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