Dylan McGeouch will be missing for up to six weeks after breaking his jaw during Celtic's 2-0 friendly loss to Real Madrid.
The 19-year-old was knocked out cold after a clash with Nura Sahin at the Lincoln Financial Field, Philadelphia.
Medical staff rushed on amid some urgent signals from concerned players on both sides.
And the forward was carried off wearing a neck brace after more than five minutes of treatment.
"He is travelling back with us which is great news," manager Neil Lennon told the club website.
"He needs a plate inserted in his jaw and I think he may made need an extraction of a tooth which has been damaged.
"Thankfully there are no neck or head injuries. It was very similar to the injury Henrik [Larsson] had when he broke his jaw against Livingston that day. So we might be looking at four to six weeks.
"Obviously it's a blow but it could have been a lot worse and he's a young lad who will recover quicker than most."
Celtic lost Anthony Stokes, who has been suffering with an ankle problem, in the warm-up, and fellow striker Daryl Murphy limped off during the first half.
There was a minor scare late in the game too when defender Thomas Rogne needed treatment for what looked like a thigh problem.
Real won comfortably with goals in each half from Jose Callejon and Karim Benzema but Celtic were up against a star-studded squad who had put five goals past both Los Angeles Galaxy and AC Milan on their US tour.