The Naomh Conaill star, arguably the most in-form player in the county at present, broke his ankle while playing for his club in the Gaeltacht Championship against Termon in Downings.
McLoone was rushed to hospital by ambulance and had to receive oxygen on the way.
An eye-witness to the horrific injury said there was no challenge on McLoone when the injury happened.
“He was going in on goal and all of a sudden he crumpled to the ground.
“You could see the bone through his sock. It didn’t look good. There’s no question he’s going to be out for a while now,” said the source at the game.
McLoone’s injury will be a big loss for Donegal boss Jim McGuinness who sees him as one of the main men in the senior team at present.
Unless the injury heals quickly, McLoone could miss the start of the championship.