The 26-year-old midfielder moved to Boro from Atalanta last summer and made 33 Premier League appearances, scoring four goals as the Teessiders were relegated to the Championship.
It always seemed likely that the former Heerenveen ace would be on his way as he has international ambitions with the Netherlands and will need to be playing top-flight football to fulfil them.
PA sport are reporting that Atalanta have agreed a fee for the midfielder they sold to the Teessiders for £12million last summer.
And while Monk has revealed that the deal is not yet done, negotiations appear to be progressing rapidly.
He told a press conference: "The update on that is we've allowed him to speak to another club and that's where it's at right now.
"Nothing is 100 per cent at this moment in time. We've allowed him to speak to another club and there are still some things we need to do in terms of incomings and outgoings."