The world and European champions looked set to extend their sensational run of competitive victories to 23 in a row when Cristiano Ronaldo put them in front at the penalty spot after 14 minutes at Mestalla on Sunday.
Valencia had other ideas, as second-half goals from Antonio Barragan and Nicolas Otamendi stunned the La Liga leaders in their first game of the year.
Second-placed Barcelona were unable to take advantage as they were beaten 1-0 at Real Sociedad later in the day, so Real remain a point clear at the top of the table.
Defender Ramos was philosophical about Real's defeat and turned his attention to getting back on track ahead of a hectic January schedule.
The Spain international told the club's official website: "There were several phases to the game and we had chances to kill off the match, but we didn't manage it.
"Valencia are an opponent who play very good football, it's a very difficult ground to come to and we mustn't take anything away from them. We have to congratulate them on their win.
"You come away from it hurting because after such a good run of wins, and being top of the table, we would have liked to get the year underway with the three points. It wasn't to be, but we'll carry on working as we have been.
"We're used to winning by a healthy scoreline in almost every match. We knew that it was going to be a hard-fought game and they gave everything they had too.
"This is where the team has to show how strong it is, it's a really tough month ahead and that motivates us all the more."
Real face city rivals Atletico Madrid in the first leg of their Copa del Rey round of 16 tie on Wednesday.
