The Brazilian winger has been suffering a knee injury since earlier this year. He went under the knife in June – three months after his last appearance and has just returned to training this week.
Martinelli's appearance at London Colney was his first since surgery.
He took part in a light first-team training session yesterday, suggesting that he may well be back ahead of schedule in a huge boost for manager Mikel Arteta.
Martinelli scored 10 goals in 26 games last season.