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:
5. Tommy Smith
Was a stalwart in Ipswich's defence a couple of years ago before moving to MLS side Colorado Rapids in 2018. The 30-year-old joined League One side Sunderland on a short-term deal last season but didn't get a chance to play for the Black Cats due to the coronavirus pandemic. Photo: SAFC
6. Chris Gunter
Boro may have decent options in the right-back position, yet Gunter seems like the type of player Warnock would want. The 31-year-old spent eight years at Reading before he was released at the end of last season and has earned 96 caps for Wales. Photo: Stu Forster
7. Tyler Blackett
After leaving Manchester United to join Reading in 2016, Blackett's four-year spell at the Madejski Stadium has been mixed. His performances improved midway through last season, yet it wasn't enough to earn the 26-year-old a new deal. Photo: Naomi Baker
8. Luke Garbutt
A player who has impressed in League One recently during loan spells with Oxford and Ipswich. The 27-year-old was on Sunderland's radar after being released by Everton but is said to have other options. Photo: Pete Norton