The 28-year-old is now a doubt for United's weekend clash with Leicester City, although Iron boss Nigel Adkins is remaining upbeat.
He said: "Grant should have played for his country in tonight's friendly with Hungary. But he reported feeling unwell with a flu-type virus and, after consulting the Northern Ireland doctor, it was decided to be fair to himself, he'd be better coming home.
"We'll assess him over the next 24 to 48 hours and keep our fingers crossed that he's okay for Saturday. But I'm always ever the optimist and I'm hoping Grant will be able to play."