We take a closer look at nine players who could have joined Neil Warnock’s side if negotiations had gone differently – scroll down and flick through the pages to see them:
1. Kieffer Moore (Cardiff)
Boro quickly identified the towering frontman as a genuine transfer target yet the Welsh international decided to join Cardiff. Moore has scored three goals in 11 league games for the Bluebirds, two of which came against Nottingham Forest. Photo: Getty Images
2. Joe Williams (Bristol City)
The highly-rated midfielder chose Bristol City over Boro after leaving Wigan in the summer. Yet Williams, 23, hasn't made a single appearance for his new club due to a thigh issue. He is expected to be back soon. Photo: Michael Regan
3. Charlie Goode (Brentford)
A player who attracted interest from several Championship clubs after winning promotion from League Two with Northampton last season. Goode, 25, has made just two league appearances for the Bees though and wasn't in the squad against Boro last time out. Photo: Alex Livesey
4. Yaya Sanogo (Free agent)
After his contract expired at French club Toulouse, Sanogo looked set to sign for Boro before the start of the season. The Teessiders pulled the plug on the deal, though, due to concerns about the player's fitness. The 27-year-old is still yet to find a new club. Photo: PASCAL PAVANI