Boro chief Aitor Karanka admitted that his players had been distracted by play-off talk and lacked the mental strength when the stakes were high.
“We need to be much stronger if we want to build something for the future ,” he said after watching his side lose to a Millwall side who lifted themselves out of the bottom three.
“What I have learned this season is that every time there has been talk and a possibility of the play-offs that we have not performed and we have lost the next match. We need to change that mentality.
“I was disappointed because we did not play at the level we did in the last four matches. Inconsistency is the biggest problem we have and that we must address.
“Millwall needed to win the game, they needed the points and their mentality was stronger. Only one team was fighting for the points and for this I was disappointed.”
Ian Holloway hailed his Millwall team after the victory.
Two first-half headers by Stefan Maierhofer put Millwall in control. Boro clawed a goal back on 79 minutes as Danny Graham was fouled on the edge on the edge of the area and Emmanuel Ledesma chipped the free-kick over the wall and under the bar.
However, Boro never looked like making it five wins on the spin and Millwall almost had a third in stoppage time but Lee Martin was flagged offside.
Delighted Holloway insisted the visitors’ win was deserved. “It’s a fantastic win. I’m proud of the players. We had certain things to concentrate on and we did. We stuck to the game plan.
“I thought the two wide men –Martyn Woolford and Lee Martin – were excellent and the big lad, Stefan Maierhofer, got us in front with two good headers.
“I was actually disappointed not to win the second-half because we had a few good chances to put the game to bed.”
Millwall are now out of the bottom three and are level on points with Doncaster, whom they host today in a game that could clinch Championship survival.
“Nobody has to tell me how important Monday’s game is,” said Holloway. “There’s no point looking at the League table now, it’s all about what it shows after the next three games.”
Middlesbrough: Konstantopoulos, Chalobah, Woodgate, Omeruo, Ledesma, Leadbitter (Main 74), Butterfield (Luke Williams 60), Whitehead (Friend 85), Adomah, Tomlin, Graham. Unused substitutes: Steele, Varga, Hines, Morris.
Millwall: Forde, Edwards, Dunne, Beevers, Malone, Bailey, Williams (Morison 62), Martin, Garvan (McDonald 85), Woolford, Maierhofer (Abdou 73). Unused substitutes: Robinson, Easter, Jackson, Bywater.
Referee: D Bond (Lancashire).