The Argentine striker was left out as the hosts began their campaign with a 2-0 victory, with Kang-in Lee and Alex Baena scoring second-half goals.
Alvarez’s absence comes amid growing reports linking him with Barcelona and Arsenal, with speculation suggesting he could seek an Atletico exit.
However, Simeone maintained that the forward simply needed additional training after his recent international commitments with Argentina.
"I deal with the sporting side. (Julian) is an extraordinary lad. We spoke, as we always do... We need more training sessions with him. He's the best player we have in attack, I've said that and I still think that. I want to build a team around him," Simeone told the Mirror.
