Cristiano Ronaldo's departure has been blamed for causing the major problems at Real Madrid.
The Bernabeu giants have lost three of their opening nine games and boss Julen Lopetegui could pay the price.
There is talk in Spain that Lopetegui will be sacked - despite him being in the job a matter of weeks.
Real Madrid are already four points behind rivals Barcelona and former president Ramon Calderon thinks the sale of Ronaldo to Juventus has caused the issues.
"The main problem beyond any doubt has been [ Ronaldo's] exit," Calderon told Love Sport Radio.
"It's impossible to replace a player like this: 50 goals a season, more than 500 goals in the nine years he was here and also a lack of planning."
"The last two years we've been suffering from that. Last year we finished 15 points behind Barcelona and we were knocked out of the cup by a lower-level team."
"It's true that we managed to win the Champions League with difficult knockout matches against Juventus and Bayern Munich but in those Cristiano was the key play for winning them so not having him is real trouble."
As well as the sale of Ronaldo, Calderon thinks the World Cup too has caused Real Madrid’s issues.
"We have to consider that the World Cup has taken its toll, mainly with players like Modric and Varane," he said.
"They played all the matches until the final and we've seen that in other teams like in Barcelona with Ivan Rakitic."
"The main problem with Real Madrid is that to win is an obligation not an aspiration. When the problems come, always the crisis appears. But we've been in a situation like this before and we've come out."
Ronaldo returns to Old Trafford for an eagerly-anticipated Group H clash against former club Manchester United.
Ronaldo was in jovial mood, flashing smiles throughout the press conference at the No.7 Lounge, where his face adorns the walls alongside George Best, David Beckham and Eric Cantona - fellow United greats who wore the shirt bearing that number.
The 33-year-old remains a fans' favourite at United after a medal-laden spell between 2003 and 2009, meaning he will not celebrate should he score in the Group H encounter.
"It is a huge, emotional thing for me to come back," Ronaldo said.
"I mean, I can remember what it was like when I was here in Manchester and know all about the history."