The former Liverpool and Catania man was hit on the head by a piece of exercise equipment, fracturing his skull.
He was initially sent home, before attending hospital where the extent of his injury was assessed, and he was subsequently placed in intensive care.
Now, the club have confirmed the player – who is in a stable condition - is set for surgery.
"Yesterday, Sebastian Leto, during routine work in the gym, suffered an accident with head trauma, leaving a wound in his scalp," the club said in a statement.
"He was treated at the club, [however] an hour after consultation with the medical team he complained of a headache and was evaluated again.
"After the evaluation it was decided he should attend hospital, where he was diagnosed with a fractured skull.
"He was assessed by a neurological team who confirmed the fracture.
"He was then evaluated by Dr. Cristian Fuster who decided he needed surgical treatment, which take place within the next 48 hrs."
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