The Teessiders have been competing near the top of the Championship for most of the campaign but now face an uphill battle to win promotion from the second tier this season.
Boro thought they had salvaged a late point at Swansea when Sam Morsy cancelled out Andre Ayew’s opener in stoppage-time, yet a controversial late penalty decision went against the visitors.
Ayew subsequently converted from the spot as the Swans moved up to third in the table and kept their automatic promotion hopes alive.
Boro will now prepare for this weekend’s home game against Stoke City – who also have an outside chance of finishing in the play-offs.
With around a quarter of the season left to play, here are the latest promotion odds from bookmakers SkyBet.