Real Madrid had Eder Militao sent off in a 2-1 defeat by Levante on Saturday, handing Atletico Madrid the chance to move 10 points clear at the top of La Liga.
Zinedine Zidane's defending champions sit second in the table, seven points behind Atletico, who have two games in hand and face Cadiz on Sunday.
Militao was shown a red card in the ninth minute at the Alfredo di Stefano Stadium, with his team already without their injured captain Sergio Ramos and Covid-stricken coach Zinedine Zidane.
Marco Asensio gave Madrid the lead four minutes later but Jose Luis Morales equalised before half-time before Roger Marti made up for missing a penalty by scoring the winner.
Barcelona coach Ronald Koeman had admitted earlier on Saturday his team "have to be realistic" about their chances of overturning Atletico's considerable lead in the table.
And with Real Madrid slipping up again, it will now take a seismic collapse for Atletico to miss out on repeating their incredible La Liga triumph in 2014.
It also leaves the Champions League as Real Madrid's only obvious chance of a trophy this season after they lost in the Spanish Super Cup semi-finals and went out to Alcoyano in the Copa del Rey. Madrid face Atalanta next month in the Champions League's last 16.
Militao was sent off for bringing down Levante's Sergio Leon, who was darting towards goal when Militao cut across him on the edge of the penalty area.
But Madrid gave themselves a lead to hang onto shortly after when Asensio continued his recent resurgence by sprinting onto Toni Kroos' brilliant through ball and finishing into the corner.
Levante were dangerous, though, and drew level before the interval as Jorge Miramon's cross to the back post was expertly caught on the half-volley by Morales.
Vinicius Junior came on for Madrid in the second half but made the worst possible start by kicking away the leg of Carlos Clerc, who was shown by VAR to have had his right foot inside the area.
Thibaut Courtois pushed away the penalty by Roger and with 11 men, Madrid might have seized the momentum.
Instead, their tired legs gave way and a well-worked short corner by Levante gave a clear sight of goal to Roger, who made amends by firing into the corner.