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Mertens has been one of Napoli's stand-out players this season, with the attacker scoring six goals in 24 Serie A appearances.
The 26-year-old has thrived since moving from Eredivisie outfit PSV Eindhoven last June, and has also emerged as a regular for Belgium.
And Callejon - who also switched to the Stadio San Paolo, from Real Madrid, ahead of the 2013-14 campaign - has tipped Mertens to shine for his country in Brazil.
"Mertens? He has great speed and great technique," Callejon told N+.
"He is very good in one-on-one (situations), he can score goals, he is already a complete player.
"I think he will do well at the World Cup with Belgium."
Belgium are fancied to progress from Group H, which also features Algeria, South Korea and Russia, at the tournament.
