The Naomh Conaill/Glenties man suffered a number of broken bones below his eye after an incident in his side’s game against Glenswilly yesterday.
McLoone had to be rushed to hospital after the game during which a massive brawl broke out involving almost 40 players and mentors.
He scored a late goal for his side in the 1-10 to 2-6 defeat to Glenswilly.
Donegal assistant manager Rory Gallagher has confirmed that the Glenties player will require surgery and will miss the Ulster decider.
A source revealed “Leo got a very bad injury. His face is a mess. The semi-final is only two weeks away and it’s doubtful if he will be ready for it
“It’s such a shame that a player who represents his county got attacked like this. A full investigation should be carried out into what happened here.”