The Teessiders have made just one signing this summer following the arrival of Grant Hall on a free transfer from QPR.
Boro boss Neil Warnock may look towards the loan market to bolster his options – so we’ve picked out nine Premier League defenders who may be available.
Scroll down and flick through the pages to see them:
1. Ben Gibson (Burnley)
An obvious one to start. It's no secret the former Boro captain wants to leave Burnley this summer to play regular first-team football again. The Teessiders are interested in re-signing the 27-year-old but a loan deal would require Burnley to pay a large chunk of the player’s wages. Photo: Alex Livesey
2. Filip Benkovic (Leicester)
A player who was on Boro's radar in January but signed for Bristol City instead. The 23-year-old will hope he can break into the Leicester first team this season, yet it will be difficult to dislodge Jonny Evans or Caglar Soyuncu. Photo: Harry Trump
3. Matt Clarke (Brighton)
The 23-year-old is back at Brighton after playing regularly during a season-long loan at Derby last term. It's unclear if Clarke will be able to step up into the Seaguls first team this season, though, especially as fellow centre-back Ben White has also returned to the Amex Stadium from Leeds. Photo: Alex Livesey
4. Dara O'Shea (West Brom)
O’Shea, 21, made his Championship debut last season and made 17 league appearances in total as West Brom were promoted to the Premier League. The Baggies will see the centre-back as one for the future but are likely to want a more experienced defensive partnership in the top flight. Photo: Shaun Botterill