The midfielder was a costly absentee from the national team during their Euro 2024 campaign after he suffered ankle ligament damage in April.
He missed the first few weeks of the season at the Catalan side, where he has made 215 appearances since joining from Ajax for €86million (£72m) in 2019.
De Jong has recently mounted a comeback for Hansi Flick's outfit, playing 15 minutes in their 5-0 Champions League win over Young Boys on October 1 before making his first La Liga appearance in six months in a 3-0 victory over Alaves last Sunday.
But Koeman, his international manager, has issued a frustrating reality check in a statement about the star's fitness, revealing that playing again was 'scary' for the 27-year-old.
He told Sport: 'I spoke on the phone last week and, with all due respect, he is still too far away [from being fit enough].
'He played a little and just that was scary for him. It was difficult for him. Frenkie needs to find his rhythm and be in 100 per cent shape and I think he still has some time to go for that.'
Last month de Jong dismissed sensational claims that his annual salary at Barcelona was an eye-watering £31m-a-year.
He told Barcelona's official website: 'It is not true. That's a very big figure and is a long way off what I really earn.'
The 27-year-old was also eager to point out that he didn't refuse to have surgery on his ankle.
De Jong insisted that it was instead a collective decision that was made between himself, the club and the doctors.
He said: 'Throughout my recovery I have chosen to stay silent.
'It is not true that the club told me I had to have an operation, and I didn't want one. Because everyone at the club, the doctors and myself all agreed that surgery was not the best option.
'It has been very frustrating for a player who lives and breathes football to be out of action for another lengthy period.'
De Jong admitted that his injuries caused him 'mental trauma' but stated that he is relishing his return to action.
De Jong has recently mounted a comeback for Hansi Flick's outfit, playing 15 minutes in their 5-0 Champions League win over Young Boys
He said: 'Having three injuries to the same ankle caused me a bit of mental trauma. But I am gradually regaining my confidence to kick the ball hard and go strong into tackles.
'From the outside, the team looks full of energy and quality. I am really keen to be part of that and offer my very best.'
De Jong missed out on the Netherlands' 1-1 draw with Hungary and will be absent once again as Koeman's side travel to Germany for a crunch match in the Nations League.