Used of the coach or manager of a soccer team, the British- and Irish-English phrase to lose the dressing room means to lose the players’ respect.

The earliest occurrences of this phrase that I have found are as follows, in chronological order:

1 & 2-: From newspaper articles about the resignation, on Friday 8^th October 1993, of the Irish former footballer William Brady (born 1956) as manager of Glasgow Celtic Football Club:

1-: From Can Macari be Kellys’ hero?, by Glenn Gibbons, published in The Observer (London, England) of Sunday 10^th October 1993:

Brady’s departure induced no tears among the hoi polloi because they had come to believe he was not officer material. A succession of expensive failures, such as Stuart Slater, Tony Cascarino, Gary Gillespie and Tony Mowbray, indicated fatally flawed judgement of players. The Irishman’s placidness was also perceived as a handicap in a job which requires infectious animation and this was confirmed by a player who said Brady had ‘lost the dressing room and when that goes, everything goes’.

2-: From Brady: Manager appeared to misjudge scale and difficulty of the job he undertook at Parkhead, by Roddy Forsyth, published in The Sunday Tribune (Dublin, County Dublin, Ireland) of Sunday 10^th October 1993:

Just as surely as he lost the supporters, Brady lost the dressing room. At one point last summer seven players, including Packie Bonner, were out of contract and unhappy with what was on offer. Peter Grant, no great footballer but a Celtic man to the bone and a player whose commitment to the club was unquestioned, found himself earning £400-£500 a week on a monthly contract.
At the same time, Stuart Slater—referred to by disgruntled teammates as “the fancy dancer”—had a take home pay of thousands. Slater, on whom Brady had spent a club record transfer fee of £1.5 million, was hived off to Ipswich Town two weeks ago for £750,000. He had scored two goals in 14 months.
All managers make mistakes in their apprentice years but there is a vast difference between making them at, say, Crystal Palace and at Old Trafford or Parkhead. The Celtic board gambled on Brady and he gambled on Slater, Cascarino, Gillespie and the rest. Both Celtic and Brady were let down.
Will he ever make a manager? If he has the resilience to learn from an experience which must be lacerating to a man of his pride, probably he can. But not with promises which cannot be fulfilled, players who cannot deliver and supporters who cannot believe that the manager understands far less about their club than they do.

3-: From Notebook, Bill Heaney’s column, published in the Lennox Herald (Dumbarton, Dunbartonshire, Scotland) of Friday 11^th August 1995:

Soccer management is a particularly demanding calling.
One minute you are a hero, the next you are yesterday’s man.
There’s little loyalty in football.
Lose the dressing room—or the terracing for that matter—especially the terracing—and you’re out the door.

4-: From Hoddle sticks by faith healer, published in the Evening Express (Aberdeen, Aberdeenshire, Scotland) of Friday 2^nd October 1998—the English former footballer Glenn Hoddle (born 1957), who was then the manager of the England national football team, had appointed Eileen Drewery, a supposed faith healer, as part of the coaching staff:

England boss Glenn Hoddle is adamant he will continue to use faith healer Eileen Drewery as part of his international set-up.
[…]
Hoddle has come under intense criticism from the media over his use of Drewery and the contents of his World Cup diary.
But he insists that all is well within the England camp ahead of the European 2000 qualifying clash with Bulgaria on October 10.
Hoddle added: “There’s a lot of rubbish written about me losing the dressing room and it’s simply not true.
“I have absolute confidence in my players and I know they are an excellent squad capable of challenging the very best in the world.
“I use a healer, so what? If I think it’s the right thing to do then I will continue to do so.”

5, 6 & 7-: From newspaper articles, published on Tuesday 8^th and Wednesday 9^th December 1998, about the sacking of the Scottish former football player Alex Miller (born 1949) as manager of Aberdeen Football Club:

5-: From Fans have their say on Alex, by Jenny Clarke, published in the Evening Express (Aberdeen, Aberdeenshire, Scotland) of Tuesday 8^th December 1998:

Neil McDougall, chairman of the Marischal Bar Reds, one of the biggest supporter clubs in the city, said: “It was inevitable, but I must admit I didn’t think it would happen until after the next three games.”
Neil reckoned the writing was on the wall over the last few weeks with appalling personal performances by some of the players.
“I think it has become obvious that Alex Miller has lost the dressing room and there is no easy way back after that.”

6-: From Sack no answer, says Jess, by Michael Grant, published in the Press and Journal (Aberdeen, Aberdeenshire, Scotland) of Wednesday 9^th December 1998:

[Eoin] Jess has played with his heart on his sleeve this season, but the performances from others in red shirts have sparked accusations that Miller had lost the dressing-room, with Dons players no longer prepared to work for him.
But Jess, on behalf of his teammates, denied that. He said: “I would say that is unfair. No player goes out not trying. I don’t think he had lost the dressing-room. We just couldn’t get out of the situation we were in. I have knuckled down this season but it is no use me playing well if we aren’t getting results.”
But Miller did not have universal support among Dons players in the dressing-room. Some felt he played players out of their best positions and chopped and changed the team too often.

7-: From the transcript of one of the telephone calls that Aberdeen Football Club supporters made to the Evening Express (Aberdeen, Aberdeenshire, Scotland), published in that newspaper on Wednesday 9^th December 1998:

Stephen Reith, 28, Orchard Road, Aberdeen: “Miller lost the dressing room but the man to turn things around would be Tommy Burns who did a great job at Kilmarnock and Celtic.”

8-: From Torment continues for besieged Gregory, by Phil Shaw, published in The Independent (London, England) of Monday 6^th December 1999—the Dutch former footballer Ruud Gullit (born 1962) was the manager of Newcastle United Football Club from August 1998 to August 1999:

Newcastle have hardly looked back since dismissing Ruud Gullit when their stock—and Stock Exchange credibility—slumped similarly and drastically.
The Dutchman had lost the dressing-room, to use the sporting vernacular.