Missing suspended top scorer Cristiano Ronaldo, Jose Mourinho's side were held to a 0-0 draw at lowly Osasuna in La Liga on Saturday and looked a shadow of the team that romped to the title last season.
They had only one shot on target at Osasuna's Reyno de Navarra stadium in Pamplona, their worst record in a La Liga game under Mourinho, and the result left them languishing in third. Unbeaten leaders Barcelona can go 18 points clear of their great rivals with a win at Malaga on Sunday.
Real's Argentina striker Gonzalo Higuain toiled ineffectively before being hauled off in the second half and even the introduction of Germany playmaker Mesut Ozil and France striker Karim Benzema failed to inspire the nine-times European champions.
Croatia midfielder Luka Modric, a high-profile signing from Tottenham Hotspur in the close season, was largely anonymous, while little-used Brazil playmaker Kaka had a night to forget when he received two yellow cards in quick succession after coming off the bench and was sent off.
Real's poor performance prompted the headline "Madrid is nothing without Cristiano" on Marca's website, while the sports daily's main rival As wrote "No firepower without Cristiano".
The Portuguese forward, who has scored 16 goals in La Liga this season and is joint Champions League top scorer with six, will be looking forward to facing his old club when United visit the Bernabeu on February 13.
On Saturday's evidence, however, if Ronaldo was to be unavailable through injury or suspension then Real's chances of winning the tie and going on to lift a 10th European crown that has eluded them since 2002 would be severely dented.
Mourinho said he was not going to use injuries or suspensions as excuses for Real's erratic form.
"I am not talking about players being out injured even though some were missing for one or two months," he told a news conference after the Osasuna game.
"Neither will I make excuses because of those who were suspended," added the Portuguese.
"I liked the team's attitude and there were players who had a great game.
"It's not easy to create chances here because of the way the opponent plays but we had opportunities to score. Not many, but we had them."
Miguel Pardeza, a former Real player and now club director, acknowledged that their chances of a second straight La Liga title were slim but said they would never give up.
"There is a big gap," he told broadcaster Canal Plus.
"But if one thing has set us apart it's that we don't throw in the towel and we keep fighting," he added. "Our reputation is at stake in every game."
The return leg of the Champions League tie against United is at Old Trafford on March 5.