Boro boss Chris Wilder has confirmed the club have accepted an offer for Marcus Tavernier.
The midfielder was the subject of a bid from a Premier League club and is close to completing the move.
Wilder said: "We have to get on with it, the club did everything to try and keep Marcus.
"When you get down to the nitty-gritty of the move and him wanting to move, I think it just became a case of negotiating the transfer fee, which is above me.
"But as I say the club tried everything they could to keep Tav, and now we have to look forward."
Tavernier is a product of the club's Academy and made 155 appearances after making his debut against Scunthorpe United in the EFL Cup in August 2017.
He scored the first of his 18 goals against Bournemouth a few weeks later in the same competition.