Diego Simeone's defending Liga champions ran riot at Vicente Calderon against their neighbours on Saturday, as two goals in either half ensured Atleti of the bragging rights for a second time in the league this season.
Assessing his side's defeat, Ronaldo was in no doubt as to the magnitude of Real's failings.
"This is a day to forget, but we are still top of the league and we know we'll have to fight right until the end," the Portugal captain told reporters following a far from pleasing return from suspension.
"We lost today and we are angry about it but we have to stay positive for the upcoming games because we only dropped three points and we're still first in the table.
"Nothing came off for us. We have to put this game behind us as quickly as possible because Real Madrid can't lose 4-0 to any team.
"We lacked everything - desire, attitude. We weren't fresh physically or mentally... but we can't make excuses. We have to keep our heads up.
"There is still plenty to go in La Liga and we are still top."
Real are now four points clear of Barcelona in second and Atletico in third, with Luis Enrique's side taking on Athletic Bilbao on Sunday.
Ronaldo added that the defeat ranked among his worst as a Real player but suggested that fatigue may have played a part following Wednesday's 2-1 win over Sevilla.
"Three years ago we lost 5-0 against Barcelona and they were difficult days," Ronaldo added.
"We haven't been able to beat Atletico in recent meetings. Things were different before but, in my opinion, we are a better team than them and we should be showing that on the pitch.
"I'm sure, in the end, we'll win La Liga. The team are a bit tired. Some of the players have been playing a lot and there wasn't much recovery time for those who were involved in Wednesday's game."
