Zinedine Zidane's side relaxed after goals from Ferland Mendy and Karim Benzema had got them off to a dream start and they nearly paid for it as Granada peppered their area in the second half after Darwin Machis pulled a goal back.
"Without realising it, we made the mistake of letting our guard down after getting a scoreline which we would have been very happy with," Ramos said.
"But it wasn't good enough and today you can't beat anyone unless you are fully focused for 90 minutes. We won the first half and lost the second and although we got the victory we did things we should never do, like conceding a cheap goal."
Goalkeeper Thibaut Courtois had to make a brilliant save late on to preserve Real's lead while Ramos blocked a shot on the goal-line, but his side recorded a ninth consecutive win to move within range of securing the title.
Real will land their first La Liga crown since 2017 with victory at home to Villarreal on Thursday before they visit Leganes on the final day of the season next Sunday.
"Winning the league would mean a lot to us, it would be the reward for the consistency we have shown and for how hard we have worked throughout the year," added Ramos, whose side were two points behind Barcelona when the season was halted in March due to the COVID-19 pandemic.
"The lockdown served us well and proved to be a turning point as we worked on preparing to return to action and set ourselves the objective of winning the title.
"Now we are in the title fight and I hope that on Thursday we can celebrate it properly."