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The 21-year-old Javi Martinez received a surprise call-up to the Spain squad after finishing his fourth full season with Athletic Bilbao. (Lluis Gene/AFP/Getty Images)

Midfielder Javi Martinez is ready to play in Spain's round of 16 match against Portugal at the World Cup on Tuesday if Xabi Alonso doesn't recover in time from his right ankle sprain.

"It's a fundamental game for the team. If I play, I'm totally prepared, ready to assume the responsibility and I'll do everything I can to do it well," Martinez said on Sunday.

The 21-year-old Martinez received a surprise call-up to the Spain squad after finishing his fourth full season with Athletic Bilbao.

Martinez made his competitive debut for the European champions as a substitute for Alonso in their 2-1 win over Chile on Friday.

He had been linked with a move to a bigger club due to his poise under pressure, strength on the ball and technical ability.

"We've got many options to contemplate," said Spain coach Vicente Del Bosque, with striker Fernando Torres also recovering from a leg injury. "I have a lot of confidence in Javi Martinez. He's going to be a key player for Spain for many years."

Martinez's selection would likely again keep Cesc Fabregas and David Silva on the substitutes bench. Martinez has made three appearances for Spain, compared to 52 for Fabregas and 37 for Silva.

Along with scorers David Villa and Andres Iniesta, Fabregas was one of the few players to create chances against Chile.

Silva has not played since the team's opening 1-0 loss to Switzerland. He was a key player as Spain won the 2008 European Championship and had been expected to fulfil a similar role in South Africa after a relatively injury-free club season.

"I know there's a lot of debates about the players ... that the midfielders are too defensive, but I'm not going to start a debate with anyone," Del Bosque said on Sunday. "The changes have amounted to very little difference. Xabi Alonso is one of our best scorers."

Spain will need its attack to be firing, as Portugal has held its three group opponents scoreless.

"Defensively, they are a very strong group, but we also know we're going to create chances," said Martinez, who doesn't think the high number of Portugal players with Spanish clubs will have a bearing on the match.

"A lot of players have a different role with the national team than with their club. Here everything is different. They know us and we know them. It starts from zero."

Spain's players are well aware of Cristiano Ronaldo's talents, and Del Bosque warned his team about the Portugal forward.

"Portugal counts on a very good team plus the value that Cristiano Ronaldo brings. It's him with a team behind him," said Del Bosque.

Spain is aiming to get further than in 2006 when it went out to France at this same stage.

"I don't feel optimistic nor pessimistic. I know football and the emotions it causes, the results it can produce."