The Teessiders have already brought in Patrick Roberts and Lukas Nmecha from Manchester City, with loan signings the most viable option. We take a look at nine Premier League centre-backs who haven’t been playing for their current clubs could be available – scroll down and flick through the pages to see them:
5. Maya Yoshida - Southampton
After starting the season in the side, Yoshida has fallen down the pecking order at St Mary's. The 31-year-old hasn't started a league game for Southampton since October. Photo: Bryn Lennon
6. Eric Garcia - Manchester City
Boro have already made two loan signings from City this month. Garcia, 19, has featured in the Premier League recently but will probably see his game time reduced when Aymeric Laporte returns from injury or if City sign another defender. Photo: Clive Mason
7. Kieron Freeman - Sheffield United
The 27-year-old full-back has was linked with Boro earlier in the season. Freeman has made just two League Cup appearances for the Blades this season. Photo: George Wood
8. Kevin Long - Burnley
Like Gibson, Long has found game time difficult to come by at Turf Moor. The 29-year-old has often been left on the bench in the league. Photo: Shaun Botterill