The 22-year-old Italian spent the majority of the last two seasons with Brentford and has failed to make a single appearance back with the Cottagers this season.
However, because Brentford are now a divisional rival of Fulham following their promotion from League One and Fulham's relegation from the Premier League, Weir doesn't think he'll have a third spell at Griffin Park.
"Marcello's been a good player for us, there's no doubt about that," Sky Sports News quotes Weir as saying.
"But I'm not sure Fulham would sanction a move to us, being one of their rivals."
Fulham's caretaker manager Kit Symons has suggested that Trotta could leave the club soon to get some first-team action under his belt.