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.”
De Roon, 26, played in last Saturday’s Sky Bet Championship defeat at Wolves, but could leave as the club attempts to balance the books after a summer spending spree.
Striker Britt Assombalonga, who cost £15million, heads a list of new signings which also includes Darren Randolph, Ashley Fletcher, Martin Braithwaite, Cyrus Christie and Jonny Howson.
Boro have recouped in the region of £10million for Gaston Ramirez following his move to Sampdoria. While, Viktor Fischer, Cristhian Stuani, Brad Guzan, James Husband and Alex Baptiste have also left the Riverside Stadium.