POTCHEFSTROOM, South Africa (AP) -- With Xabi Alonso in doubt for Spain's round of 16 match against Portugal at the World Cup, his likely replacement is ready to step in.

Midfielder Javi Martinez made his debut for the European champions in their 2-1 win over Chile on Friday after replacing Alonso, who sprained his right ankle.

Martinez says he will "try to do the best I can'' if selected for Tuesday's match at Cape Town's Green Point Stadium.

Coach Vicente del Bosque says the 21-year-old Martinez is "going to be a key player for Spain for many years.''

Martinez's selection could leave Cesc Fabregas and David Silva on the bench.

Del Bosque may make other changes with striker Fernando Torres also recovering from a leg injury.

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