The Brazilian teenager has been a revelation since joining from Ituano last summer, having scored 10 goals in 26 appearances.
But after damaging the cartilage in his left knee during a training session on June 21, Martinelli underwnt surgery and will now recover until the end of the year.
A statement on the club's website reads: "Surgery was successful, with consultants and our medical team very encouraged with Gabi's current progress at this stage of his recovery.
"[He is] currently at our training centre every day receiving specialist attention and support from our medical team. [He] will continue his recovery throughout the close season, with the aim of a return to full training by the end of 2020."