The defender said the loss at the New Den on September 16, Leeds’ first league defeat after a seven-match unbeaten start, showed rival clubs how to play against them and exploit their lack of Championship experience.
United were a month-and-a-half into the term when an impressive run was unceremoniously ended by a one-sided clash at The New Den, a venue where Leeds typically struggle to pick up points.
The defeat marked the beginning of a run of seven losses in nine league matches and despite a flurry of improved results leading up to Christmas, United’s form deteriorated again after the turn of the year and led to the sacking of former head coach Thomas Christiansen in February.
Christiansen’s replacement, Paul Heckingbottom, failed to arrest Leeds’ slide down the table and three wins from his 15 games in charge have left the club in 14th position ahead of their last game at home to Queens Park Rangers on Sunday.
United recruited heavily from abroad during last summer’s transfer window and Cooper said the outing at Millwall had been a harsh lesson in Championship football. Leeds’ brittle nature has been underlined by a tally of 20 league losses.
Cooper said the impact of the visit to the New Den became apparent in subsequent games. “At the time, no you don’t (think it will change the season) but looking back I think it did, the way we got beat,” he said.
“We started so well and it hit us really hard. We were bitterly disappointed that was couldn’t turn around, bounce straight back and go on one of those runs again.
“The Den’s a very hard place to go and I think that was definitely a starting point of what the Championship’s about. Teams, I think, after that game set up a lot differently against us. They did try and bully us and maybe we weren’t used to that. I think we’d definitely be used to that now. We know what the Championship’s about.
“The lads who’d come in, it was maybe an eye-opener for them. I think it’ll stand us in good stead for next year.”
Cooper described Heckingbottom as “a breath of fresh air” despite criticism of the 40-year-old’s impact as head coach and the sequence of disappointing scorelines under him. Last weekend’s 2-1 defeat at Norwich was the eighth of his short tenure.
“He wants to play the right way and I’m sure he’ll set his stamp next season,” Cooper said. “He’s a very good coach who gets the players motivated.
“He tells us if we’re not pulling our weight and he’s been a pleasure to work with but we’ll be judged on results and we’ve not been getting results. We need to turn that around.
“It’s hard to put your finger on it but we’ve got to take a lot of the blame, the players. We set up in the right way, we want to try and play football and we’ve brought a more disciplined approach to our game. You could say it’s luck but in this game you earn your luck and we’re not doing that.
“To go one up at the weekend and get beat was so disappointing. We can’t go along like that. We can’t be a soft touch. If you want to do anything in this league, and if you look at the teams who’ve succeeded this year, they’ve all got that winners’ mentality.”
Cooper defended the attitude of United’s players - something questioned publicly by owner Andrea Radrizzani in March - but said the squad were aware that changes were inevitable in the summer transfer market following a disappointing season.
“That’s just the reality of football,” he said. “The new manager’s going to have his own ideas. It’s hard but that’s the tough reality of the game. That’s the way it is.
“I don’t think there’s one player in the squad who goes out with a bad attitude, who wants to have a bad game or wants to lose. That’s not in the lads. Sometimes it doesn’t fall for you but you earn your luck in this game.
“Personally I can’t wait to get in for pre-season already. During the season you’ve got games to put it right but now we’re going to dwell on this all summer. I don’t think there’s one lad in there who won’t dwell on it.”