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:
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.