The defending champions lost 2-1 at home to Levante on Saturday, meaning they remain seven points behind the league leaders, who now have two games in hand.
Madrid led through Marco Asensio after 13 minutes but were already down to 10 men by that stage, Eder Militao dismissed following a two-minute VAR delay.
Timed at eight minutes and 12 seconds, it was the quickest red card received by a Madrid outfield player in LaLiga since at least the 2003-04 season.
Los Blancos could not withstand the Levante pressure and were pegged back before half-time by Jose Luis Morales.
Courtois saved a penalty – his second such stop for the club – after the break, but Roger Marti, the man denied from 12 yards, struck the winner late on.
The setback represented a big blow to Madrid, who had been enjoying their best home run of a tricky season with four straight wins.
Meanwhile, Atletico, who visit Cadiz on Sunday, have won seven in a row in LaLiga.
"We have to fight. LaLiga lasts until May," Courtois said. "We have to react and hope Atletico lose, but they keep winning.
"Today we cannot consider it a bad game or a good game – we had to play with one less."
Courtois was not convinced by the red card for Militao, one of just two first-half Madrid fouls.
The defender was initially shown a yellow card as he tangled with Sergio Leon, but the referee headed to the pitchside monitor and considered the retreating Raphael Varane to have been taken out of the game by the forward's touch.
"I don't think he means to knock down the player," Courtois said. "And Varane is close. Although [Leon] has a chance, I don't think it's a red."
Courtois added Madrid "missed something" in attack, failing to aim a single shot on target in the second half while their goalkeeper made six saves across the 90 minutes – his most in a single game for the club.
"I want to highlight our game, which was difficult after we were left with 10," Bettoni said. "We showed character and a lot of pride.
"The fans have to continue believing in their team. We will fight to the end for the remaining titles.
"The effort of the players has been great and that is what matters to us, although obviously we are disappointed by this defeat."