Álvarez poked home a cross by Antoine Griezmann to give Diego Simeone's team the win in Vigo on Thursday.
"What matters are the three points," said Álvarez, Atletico's big-money offseason signing.
"If it's 1-0 or 5-0, what is important is to get the three points. We knew it was going to be a difficult match against Celta here."
Atletico remain two points behind second-place Madrid going into Sunday's match at Atletico's Metropolitano Stadium. Both are chasing Barcelona, who have won their first seven league games this season.
Atletico struggled to create scoring opportunities until Griezmann curled a left-footed cross into the area. Álvarez ran between a couple of defenders to poke the ball into the net with his right foot.
It was the 24-year-old Argentina forward's second goal since joining Atletico from Manchester City.
Celta, who have lost four of their last five matches, had come close to scoring several times but were stopped by yet another solid performance by goalkeeper Jan Oblak.
Atletico were coming off a 1-1 draw at Rayo Vallecano, but had won their previous three games in all competitions.
After the derby at the Metropolitano, Simeone's team will visit Benfica in the Champions League on Wednesday.
Meanwhile, Ayoze Pérez scored twice as Villarreal came from behind to win 2-1 at promoted Espanyol and stay near the top.
Pérez has scored six goals in seven matches, one fewer than the league's leading scorer, Barcelona's Robert Lewandowski.
It was the fourth win for Villarreal, whose only loss was a 5-1 rout at home against Barcelona in the previous round.
Spanish media said the referee of the match had been hit by an object thrown from the stands while he left the field after the final whistle.
Last-place Las Palmas drew 1-1 with Real Betis to remain winless after seven rounds. They haven't won in 21 straight league matches going back to last season, when lucky to escape relegation.
The hosts scored first with Alberto Moleiro in the ninth minute and 11th-place Betis - coming off two consecutive victories - equalised through Giovani Lo Celso in first-half stoppage time.
Getafe are the only other team without a league win this season.
