The 20-year-old damaged an ankle in a collision with 6ft 7" giant goalkeeper Fraser Forster during Friday night's friendly victory at Darlington.
It cost him a place in the first-team squad when they flew out to the United States on Sunday.
Ferguson is due to have a second scan on the injury today, and like midfielder Danny Guthrie, could miss the start of the new season.
Shane Ferguson: "It all happened so quickly.
"I tried to get out of Fraser's way and didn't manage it quick enough.
"My leg got caught underneath him and it's unfortunate really - just bad timing.
"I knew straight away that something wasn't right. They had a look at me and they think it is the ligaments in my ankle.
"I've had another scan and I will just see how it is then and how long I will be out for.
"I'm absolutely gutted [at not going to America]. It's the worst timing ever.
"I have just got to try and get fit now, hopefully before the start of the season and if not, then just a little bit after."
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