The forward is back on Tyneside after suffering a hamstring problem at the club’s pre-season training camp in Ireland.
De Jong returned home early from Dublin this week.
The Holland international’s career at St James’s Park has been punctuated by a series of injuries.
And the luckless 27-year-old – who has been ruled out of this afternoon’s game against Bohemian in Dublin – is now being assessed ahead of the rest of the club’s pre-season programme.
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Asked about de Jong’s injury, United manager Rafa Benitez said: “He was a really bad moment to be fair.
“I was close to him and he tried to keep the ball in with his heel, and there was a problem with his hamstring.
“That can sometimes happen. It was a bad movement, but it was a positive thing in terms of trying to get the ball and fighting for that.
“That’s the positive view, but the negative is that the movement was too dangerous.
“Now we have to wait to see if he is fit.”
Meanwhile, defender Kevin Mbabu has also returned to Tyneside for treatment after suffering a knock in Ireland.