Playmaker Mata spoke to the media for the first time on Monday following Saturday's 37.1 million pounds ($61.22 million) move.
The club revealed he would wear the number eight shirt rather than the number seven used by former greats David Beckham, Cristiano Ronaldo and Eric Cantona.
The 25-year-old Mata struggled for game time this season under Jose Mourinho, with Chelsea's manager preferring an attacking midfield trio comprising Brazilian internationals Oscar and Willian and Belgium winger Eden Hazard.
That led to fears that if he continued to be overlooked he would struggle to claim a seat on the plane to Brazil.
Mata said getting regular game time was crucial ahead of the June finals. "It was really important," he told a news conference ahead of Tuesday's Premier League match with Cardiff City.
"As you know the Spanish squad is really difficult to get in.
"Lots of players are fighting to be in the last 23 going to Brazil so I hope this is going to be a great chance for me to keep playing, to keep being fit and to keep showing to the manager of the national team that I want to be there and I have to be there."
Mata would not say that remaining at Chelsea would have cost him his Spain place.
"I don't know, now I'm here, now the moment is here, now I'm trying to play as much I can to try and help the team," he added.
"Obviously if things are good in the last part of the season the World Cup is going to be closer for me for sure."
Spain have drawn Netherlands, Chile and Australia in Group B.
(Reporting by Josh Reich; editing by Tony Jimenez)
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