Lionel Messi scored four goals and Barcelona reclaimed top spot in La Liga as Real Madrid suffered a surprise defeat by Levante on Saturday, a week before the Clasico.
Madrid's nightmare afternoon was made worse by another injury to Eden Hazard, who was forced off in the second half of their 1-0 defeat in Valencia and is now doubtful to face Manchester City on Wednesday in the Champions League.
"It doesn't look good," said Real coach Zinedine Zidane afterwards.
Messi, meanwhile, ridiculed talk of a drought by scoring four goals against Eibar after four games without one, while emergency signing Martin Braithwaite made two assists off the bench, teeing up Messi and then Arthur Melo in injury-time.
Their 5-0 rout, coupled with Madrid's defeat, means Barca move back to the summit of La Liga, two points ahead of Zidane's side ahead of next Saturday's showdown at the Santiago Bernabeu.
"We've had good times this season and now this is a bad time," said Zidane.
"Now we have a week where everything is at stake."
It amounts to a considerable shift in momentum, with Barcelona away at Napoli in the first leg of the Champions League last 16 on Tuesday.
Madrid host City a day later and the plan had been for Hazard to regain form and fitness in what was only his second start since returning from three months out with a broken right foot.
But the Belgian sat in the dug-out with an ice pack around the same foot after going off and Zidane suggested it was a repeat injury after the match.
"It can be weak where you've had an injury," he said.
"It's sore now but we will see tomorrow after more tests."
Madrid were shaken by Hazard's departure and Levante capitalised as Jose Luis Morales banged a shot into the top corner from the angle, even if Thibaut Courtois pulled out of a save he should have made.
For Madrid, playing catch-up again next weekend will be a particularly heavy psychological blow, especially given the series of off-field problems engulfing Barcelona in recent weeks.
Yet on the pitch, Messi showed no sign of distraction as the 32-year-old completed the second fastest of his now 36 league hat-tricks, after less than 40 minutes at Camp Nou.
"There's nothing left to do but stand up and applaud," Eibar wrote on Twitter afterwards.
"I won't wash my kit after hugging Messi," said Braithwaite.
Braithwaite's arrival from Leganes drew criticism after Barcelona capitalised on a curious La Liga rule that allows clubs to sign outside of the transfer window if they have lost a player to serious injury.
Leganes, without their top scorer, were beaten earlier by fellow strugglers Celta Vigo but Braithwaite made the perfect start with his new team.