The initial fee could be around £15m plus add-ons.
The defender spent last season out on loan at Nottingham Forest.
“Discussions for the right-back, who spent last season on loan at Nottingham Forest, are centred around an initial fee of £15m plus add-ons.”
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Meanwhile, Boro boss Chris Wilder has been talking about his recruitment plans this summer.
"I'm signing players now that I believe fit in with the thread of the football club," Wilder told BBC Radio Tees.
"I've said all along the club was certainly an incredible fit for me, and I like to think myself and my coaching staff are a good fit for the football club.
"Now we've got to do that in terms of the players that are coming in."