The Eredivisie outfit have identified the teenage forward as the ideal man to replace Luuk De Jong, who moved to Borussia Monchengladbach on Wednesday
It was reported on Thursday morning that the Dutch side had agreed personal terms with the player and a transfer fee with the Nerazzurri.
However, Munsterman explained that while former the Feyenoord forward has his heart set on a switch to De Grolsch Veste, he still has to thrash out a deal with Inter.
"Luc is a serious option for Twente," he told Eredivisie Live. "He wants to come to us and we want him to come to Enschede. But first, we'll have to contact Inter about this."
The 19-year-old Castaignos only arrived at San Siro from De Kuip last summer but he managed just six appearances and one goal during the 2011-12 Serie A campaign.