The 26-year-old Holland international was rushed to hospital with blood pouring from the eye after a training ground accident on Wednesday, and there were genuine fears that he had suffered permanent damage.
However, while he faces up to two weeks on the sidelines, a relieved De Jong has been told he should make a full recovery.
Head coach Steve McClaren, who was attending the memorial service for former Magpies goalkeeper Pavel Srnicek at the time, said: “It was just in training and he accidentally got poked in the eye.
“He went down screaming, so they said ‘Let’s have a look at your eye’ and there was blood pouring from it. He wears contact lenses, and one of the contacts had come out and gone into his eye.”
Asked if there was a fear de Jong could have been blinded, McClaren replied: “Yes, yes, a massive danger, a big danger. He was rushed to hospital.
“I was just back from the memorial and I got a call saying there’d been an accident in training, so I was thinking, ‘Which one?’ and it was Siem. But by the time I got back, he was here (at the training ground) and the doc said, ‘It’s not too bad — but it looks bad’.
“I’ve seen him this morning and it looks a lot better. Fortunately, the doctor says [it needs] probably a week or two to settle down, so he’s very lucky.”
The accident is the latest misfortune to hit de Jong, who has made just 22 appearances, only seven of them starts, for the club he joined from Ajax during the summer of 2014.
His fitness problems to date include a collapsed lung — the second of his career — which sidelined him for two months last season.