The Easter Road centre-back was left with a black eye following a clash of heads with Celtic striker Amido Balde during the 1-1 draw in Leith last week.
But further tests revealed the injury was much worse than first feared and he now faces several weeks out of action after he was forced to undergo surgery to repair the damage.
The 33-year-old - who missed Saturday’s 2-0 defeat to Aberdeen - has had two metal plates inserted to protect the fracture but he claims he may also have to wear a protective mask once he is passed fit to play.
Nelson used his official Twitter site to confirm news of his lay-off, posting: “Well, that black eye turned into 3 fractures an operation and 2 metal plates in my cheekbone #AllFixed #OnTheMend #HateBeingInjured.
“Can’t do anything for a while so I’m frustrated. Hopefully won’t be out too long.”
He added: “I’m not sure how long I will be out for at the minute but I hope to get back wearing a face mask but it will still be a few weeks.”