Morais suffered "severe concussion" in the three-vehicle collision in Sheepfoot Lane, near Oldham's home ground, Boundary Park, the League One club confirmed.
Morais was forced to miss Oldham's derby clash at Rochdale following the crash at 12.50pm.
In a statement on their website, Oldham Athletic said Morais would be detained in the Royal Oldham Hospital overnight.
The statement added: "The forward was involved in a three-vehicle road accident on Sheepfoot Lane, not far from Boundary Park, shortly before the team left for their npower League One match at Rochdale."
Morais, a former Portugal Under-21 international, joined Oldham in 2010.