The Spanish champions signed Siqueira from La Liga rivals Granada in June and, having sold Filipe Luis to Chelsea, also added left-back Ansaldi on a season-long loan deal from Zenit.
Siqueira believes Ansaldi's arrival can only be a good thing as Diego Simeone's side target more honours this season.
"I can't see myself as a starter, because I am also new to the team." he said at a news conference. "I am learning and Cristian surely comes to do the same. He comes to add [to the squad].
"We are pleased with his arrival because here you have to look at the collective.
"I always say 'look at the group' and we're both here to help. The position will be well covered and [having] two [players in the position] is going to be good for Atletico."
Atletico secured the league title last season and also reached the final of the UEFA Champions League.
And Siqueira is confident the team have the right mentality to challenge on all fronts once again.
"The team is well reinforced," he added. "The club is trying to bring people who can deliver. We must follow the club's philosophy. Fight for everything, always playing match-by-match, without refusing anything.
"The team has to try to do their best and get as far as possible in all competitions they compete in."
