The Italian striker has spent the majority of his time with the Whites on loan at Wycombe Wanderers, Watford and more recently Brentford.
Despite admitting that there are some good aspects to the 20-year-old's game, Jol has challenged him to enhance his skills even further ahead of the new Premier League campaign.
"Trotta is someone who scored goals at Wycombe, now he went to Brentford and made a step up again," the Dutch boss told the Fulham Chronicle. "It's not the Premier League, it's important they play games and make progress. Alex Kacakilic and Kerim Frei did that, while Lauri Dalla Valle didn't, so he's at another club now.
"Trotta is not always in the team, it's a tough league but not the Premier League. For me, it's important they show the right mentality to play and prove themselves. Trotta is a goal poacher but there's other areas of his game he needs to improve."
Trotta missed a stoppage-time penalty last weekend which cost Brentford the chance to go up to the Championship via automatic promotion.