So far the Teessiders have only added defender Grant Hall to their ranks this summer, with manager Neil Warnock looking to strengthen in all departments.
Despite the arrival of Hall, Boro are still looking to bolster their defensive options and have been chasing Northampton centre-back Charlie Goode.
Yet the Cobblers have insisted they don’t need to sell their captain following promotion to League One and are standing firm with their valuation.
Warnock may also be looking to add more full-backs to his squad after deploying a wing-back system at times last season.
With that in mind, we’ve picked out nine defenders who are available as free agents – scroll down and flick through our picture gallery to see the full list:
1. Eric Lichaj
The experienced defender has plenty of Championship experience after spells at Nottingham Forest and Hull. The 31-year-old can play anywhere across the back four and captained the Tigers last season. Photo: George Wood
2. Mike Van Der Hoorn
May be out of Boro's price range after joining Swansea while they were in the Premier League in 2016. The Dutch centre-half, 27, played regularly for the Swans when fit in the Championship but did miss part of last campaign with a knee injury. Photo: Alex Pantling
3. Cuco Martina
Fell down the pecking order at Everton last season . The 30-year-old spent three years at Goodison Park, which included loan spells at Stoke and Dutch side Feyenoord. Photo: Michael Regan
4. Jordan Obita
Someone who can play at left-back and as a winger. Marvin Johnson played as a left-back under Warnock last season but has played as a winger for most of his career. Obita, 26, could provide more competition for places on the flank. Photo: Dan Istitene