The world champions were 1-0 victors over France, recovering from consecutive home draws which had raised the prospect of having to qualify for the 2014 World Cup through the playoffs, to return to the top of Group I.
A hand injury to goalkeeping captain Iker Casillas and a hamstring strain for full back Jordi Alba meant habitual bench-warmers Valdes and Monreal were called upon to perform at the Stade de France.
The replacements were two of Spain's outstanding figures, alongside the restored midfield engine room of Xavi, Xabi Alonso and Sergio Busquets, as the side kept a clean sheet.
"The new heroes," sports daily Marca dubbed Valdes and Monreal on Wednesday.
"The two played for the first time in a big game for Spain and they performed to a spectacular level.
"Thanks to their contributions, Spain took a giant step towards appearing at the World Cup next year."
Valdes has three Champions League titles under his belt with Barcelona and is hardly a novice, but was filling in for the hero of Spain's Euro 2008, 2010 World Cup and Euro 2012 triumphs, and a man known as 'Saint Iker'.
Casillas was watching from the stands as Valdes, 31, won only his 13th cap and pulled off a couple of spectacular saves to keep Spain on course for victory.
Arsenal left back Monreal, 27, was winning only his 12th cap, like Valdes mostly earned from friendlies and low-key matches, and he provided the cross for Pedro's winning goal.
"We have a settled body of players in the team, consistent and stable, and those who step up are rising to the challenge," Spain coach Vicente del Bosque said.
"One of the biggest problems for coaches is to give those who deserve it a chance, and we cannot always do that."
The former Real Madrid boss valued the attitude of the players and how they stuck to Spain's tried and tested style of patient possession football, despite a few days of uncertainty following their shock 1-1 draw at home to Finland last Friday.
"They have been uncomfortable, difficult days. The players have shown great maturity," he said.
"We arrived in Paris aware that a defeat would leave us five points adrift, but we did not lose our patience. We played in our most orthodox way, loyal to our idea and style."
With remaining games away to Finland and at home to Belarus and Georgia, Spain top their group by a point from France.
"There are three rounds of games left and many things could still happen," Del Bosque said.
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