Goals from Britt Assombalonga and Jonny Howson condemned Millwall to a 2-0 defeat at the Riverside Stadium against Middlesbrough on Saturday evening.
The loss - Millwall's first on the road since New Year's Day - ensures that The Lions' chances of reaching the Sky Bet Championship Play-Offs are now merely mathematical.
Neil Harris was forced into one change to his starting XI, with the injured George Saville being replaced by Ryan Tunnicliffe.
Conor McLaughlin, Shane Ferguson and Ben Thompson were named among the substitutes.
The Lions soaked up early pressure from the home side, who fashioned their first chance of the evening with five minutes on the clock - Howson, however, headed over the crossbar.
On 12 minutes, after a brief spell of pressure from Millwall, Middlesbrough took the lead. Assombalonga's original effort was blocked by Mahlon Romeo, but he was not to be denied at the second attempt, striking through Shaun Hutchinson's legs and past Jordan Archer off the post to open the scoring.
The Lions started to get the ball down on the floor and look to pass their way to an equaliser following that, but the final ball let them down as the midway point of the first half approached.
Muhamed Besic's run on 28 minutes looked to have produced a chance for Stewart Downing, but the former England international scuffed his strike wide. A further shot from Downing a minute later was deflected behind by Tunnicliffe - Daniel Ayala headed behind from the resulting corner-kick.
Romeo had to withdraw through injury shortly before the half-time break, whilst Hutchinson had to be at his defensive best on 40 minutes as he blocked a goal-bound Downing effort.
In first half injury-time Morison looked to turn a corner-kick towards goal and - despite his shirt being pulled in the process - managed to do so, but no Millwall man was able to guide the ball home.
As The Lions' luck worsened, Ben Marshall became the second Millwall player to withdraw - this time on a stretcher - through injury less than 10 minutes after the resumption, with Tom Elliott his replacement.
Assombalonga pounced on some slack Millwall defending just after the hour mark and slid in Traore, but a fantastic last-ditch challenge from McLaughlin prevented Middlesbrough from doubling their lead.
The Lions' first real opening came shortly after, but following Gregory's flick, Wallace was unable to get to the ball first inside the six-yard box. Hutchinson's looping header was claimed by Darren Randolph from the corner that followed.
On 66 minutes, a real defensive mix-up handed the home side their second goal. A cross was not dealt with by a combination of Archer and Jake Cooper, with the ball dropping to Howson a few yards out. The midfielder had the simplest of tasks in tapping home from close range.
Wallace rolled a shot into the arms of Randolph with 78 minutes on the clock, whilst Shaun Williams met the winger's cross with a powerful header three minutes later, but straight into the stopper's grasp.
Millwall huffed and puffed late on, but were unable to find a route to goal.
Middlesbrough: Randolph, Friend, Ayala, Shotton, Gibson, Clayton, Assombalonga (Bamford 77'), Howson, Downing (Fabio 81'), Besic, Traore. Subs not used: Dimi, Leadbitter, Cranie, Fry, Harrison.
Scorer: Assombalonga 12', Howson 66'
Booking: Friend, Besic
Millwall: Archer, Meredith (Ferguson 83'), Hutchinson, Williams, Gregory, Romeo (McLaughlin 38'), Wallace, Tunnicliffe, Morison, Cooper, Marshall (Elliott 54'). Subs not used: Martin, Thomspon, Onyedinma, O'Brien.
Booking: Williams, Hutchinson, Tunnicliffe
Referee: Paul Tierney
Attendance: 28,606 (1,095)