The young Brazilian has battled for minutes so far this season, despite being fully fit.
The Mirror via Fotboll Transfers says he is starting to get frustrated with the limited playing time.
He has been left on the bench for 90 minutes in the last five matches.
Martinelli had a shortened preseason after being part of Brazil's gold medal winning Olympics squad. The striker has struggled with a knee injury over the past year, though fought back to play in the final weeks of last season. A goal in victory over Sheffield United in April was his first in 15 months.
The 20 year-old's contract runs to 2024, with the option of another 12 months.
- updated October 6
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