To “lose the dressing room” is for a coach or manager to lose the confidence and support of the players. The dressing room is the changing room, where the manager might address a team before or after a game. The saying is used in association football in the United Kingdom, but has also been used in soccer in the United States.
 
“Lost the dressing room” has been cited in print since at least 1993. 
 
     
Wikipedia: Glossary of association football terms
Lost the dressing room: where a team’s manager is deemed to have lost control and support of the players.
 
Sports Pundit—Football terms
Lost The Dressing Room
The term “lost the dressing room” is a footballing term that is used to describe a situation where the manager of a football club has lost all confidence of his players and is on the verge of getting the sack. The term gains application at the time when the manager is under a lot of pressure due to the team struggling on the pitch and when he has lost the confidence, support and faith of his players in the dressing room.
     
24 August 1996, Toronto (Ontario) Star, “...The Other Coaches,” pg. E5:     
If the Swedes are slow off the mark, then Forsberg will quickly lose the dressing room.
 
Google Books
The Official Dictionary of Unofficial English
By Grant Barrett
New York, NY: Oxford University Press
2006
Pg. 218:
lose the dressing room v. of a sports team manager or coach, to not have the respect of the players. Sports. United Kingdom.
1993 Glenn Gibbons Observer (U.K.) (Oct. 10) “Can Macari Be Kellys’ Hero?”: The Irishman’s placidness was also seen 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.”
 
Google Books
Once in a Lifetime:
The Incredible Story of the New York Cosmos
By Gavin Newsham
London: Atlantic Books, Ltd.
2006
Pg. 117:
Today, it would be said that Bradley had ‘lost the dressing room’. In the world of the Cosmos, though, it was pretty much par for the course.
 
Google Books
Football English:
Soccer Vocabulary for Learners of English
By Tom Challenger
Vienna: Eniko Books
2012
Pg. 102:
To lose the dressing room: if a manager/coach has ‘lost the dressing room’ it means that the players no longer respect or listen to him It is therefore very difficult for the manager to motivate the players and to make them play with his tactics. (The dressing room is the room in the stadium where the players change into their football kit before the game.)
 
Google Books
Northern Ireland:
The Reluctant Peace
By Feargal Cochrane
New Haven, CT: Yale University Press
2013
Pg. 50:
In footballing parlance he had ‘lost the dressing room’ and he resigned shortly afterwards, to be replaced by his cousin, James Chichester-Clark.
 
The Telegraph (UK)
Jose Mourinho lost the dressing room at Real Madrid and failed in the transfer market
The standards by which Real Madrid managers are judged mean Jose Mourinho’s time at the club was considered a failure. He failed to permanently topple Barcelona, and he failed to deliver the club’s 10th European Cup.
By Pete Jenson, Madrid
2:30PM BST 03 Jun 2013
Ask what lies behind this underwhelming legacy, and the answer has generally been that new Chelsea manager Mourinho lost the dressing room.