The Brazilian born man can play in either full back position, but has been playing in the left back role since signing for the Bluebirds from Manchester in 2014.
Fabio, brother of Lyon right back Rafael, played with his brother at Old Trafford for 6 years before moving on to new pastures.
A fee of around £2 million is rumoured to be the price Middlesbrough will pay, and local paper The Gazette believes that negotiations between both clubs and the player's representatives are ongoing.
It has been no secret that Aitor Karanka has been looking to bolster his backline, and Fabio looks to be an answer to his problem at left back after James Husband sustained a shoulder injury in Boro's first pre-season friendly against York.
How will Fabio fit in to Boro's team?
Fabio has, undoubtedly, been brought in to provide back up and competition in the full back positions. Karanka has Nsue and Barragan as his options for the right hand side of defense, with the latter looking more and more likely to start the season alongside Ben Gibson as kick-off comes ever closer.
George Friend, however, is the only fit left back at the club following the injury to Husband. It looks likely Fabio is seen as back up to Friend and could play right back if called upon, a smart move by Karanka as and injury to Boro's vice captain Friend could spell defensive trouble.
Karanka, however, may have been looking at another left back even before Husband's injury as the former Doncaster full back has been loaned out to Fulham and Huddersfield in the past. He was not in the Spanish coach's plans in the Championship and may not have been in his plans as he prepared for the top flight.
Are Middlesbrough still in the transfer market?
Aitor Karanka is still on the hunt for another centre back as he looks to build on the strong defensive unit that was the best in the Championship last year.
After Boro's transfer of Neven Subotic fell through, many centre halves have been said to be interesting Middlesbrough. One defender linked to the Teesside club was Hull City's Harry Maguire.
The former Sheffielf United centre back was reportedly the subject of two Boro bids, but yesterday The Gazette reported that Middlesbrough had 'grown frustrated at the lack of responce from Hull' and have now set their sights on 'two other potential rearguard recruits,'
Darlington based newspaper, The Northern Echo, have said that one of the players Boro are interested in signing is Wales centre back James Chester. They claim Boro are 'in discussions with West Brom over the purchase of £5m-rated Chester.'