City forward Alvarez was a key player in Argentina's triumphant campaign at the tournament in Qatar, coming into the starting line-up after the first two group games and going on to score four times.
After playing in the final on December 18, the 22-year-old returned to City's training ground last Friday and a day later came off the bench late on in the 1-1 Premier League draw with Everton at the Etihad Stadium.
Guardiola said of Alvarez: "In the last minutes, at 1-1, we needed people in the box. That is why he played.
"But in the end, he rested one week, 10 days - it's not much after the World Cup final."
Alvarez's World Cup goals added to seven he has scored for City this term. He has made 21 appearances for the club, 13 as a substitute.
Guardiola was asked how much consideration he had given to playing Alvarez and City's 27-goal top-scorer Erling Haaland - who put them ahead against Everton - together against teams that defend deep, and he said: "Yeah it can happen. The ball arrives there, and you have two strikers that are going to score goals.
"But to create these chances, you have to make the process, and sometimes you need other types of players to create these chances.
"But of course, they can play together, especially against five at the back."
Another player introduced late on against Everton was Phil Foden, who has been utilised in that way in all three of City's games since the World Cup. In both of the two league matches, he has come on for fellow England international Jack Grealish.
Guardiola said Foden made "incredibly good contributions" against Everton, and added: "Phil can play in a thousand positions.
"I just, on the pitch, in the training sessions, I see something, my intuition, and I decide to play with Jack (Grealish) in these games because he gives us extra, extra passes. When it's more vertical, Phil is better. Both can combine, both can play together."
Guardiola also spoke of the difficulty he has in general picking teams from the squad at his disposal, and how player body language is a key factor in his selection.
"Every time it is difficult to choose for me, because always top players are not playing," he said.
"Lately, I look at body language, in the training sessions and everything. Because you cannot play good when the body language is not correct.
"Sometimes I choose the players for the body language, how they train, how happy they are. And this is one of the main decisions when I choose the line-up, the body language of the players. With the skills, I know how good they are. The body language depends on them, and sometimes they are not good and it is more difficult."
City, seven points behind leaders Arsenal in second place, are back in top-flight action on Thursday with a trip to Chelsea, and also play Manchester United away and Tottenham at home in January.
Guardiola said of that sequence: "That's good. Listen, in the Premier League, you play with all the teams. We try to improve, to get better, and we are going there to win the games."