The 19-year-old joined Inter from Feyenoord at the start of the season, but has so far struggled to impose himself in Italy, with Treffers suggesting that a move away would be beneficial to his client.
"Inter said that they wanted to rejuvenate their team when they signed Luc, but the average age is over 30 now. I think that a loan move is the most realistic option, or perhaps a big club will be willing to take him on a permanent move," Treffers told NUsport.
"Next season will be crucial for Luc. He wants regular first-team football, and that seems to be difficult at Inter. We'll see what happens, but there's plenty of interest."
Castaignos has made just six Serie A appearances this season.