Piazon, on loan for a second season from Chelsea, had to be carried off the field during the first half of the 0-0 draw at Elland Road, with manager Slavisa Jokanovic later confirming the worst.
"It is a very bad injury and we are going to lose him for a long time," the boss explained.
Having immediately left the ground for hospital, it is believed that Piazon has a fractured fibul
Jokanovic at least expressed hope that the 23-year-old will return to action when fit.
"We are so disappointed with the situation about Lucas Piazon, I expect he is going to recover soon and he will be available soon," the ex-Chelsea player said.