They've already signed Ander Herrera and Luke Shaw, but Manchester United's summer spending seems to be far from over.

The Red Devils are looking to rebuild under Louis van Gaal, and get back to the winning ways they and their fans have got so accustomed to. To do so, the Dutch manager looks set to bring in fresh faces at Old Trafford, to make this his own United squad. A new defender and midfielder already brought in, van Gaal has now been heavily linked with a new striker.

Porto's Jackson Martinez has caught the eye of many European football fans this season with his fantastic goal tally. The striker bagged 20 goals last year, making it 46 league goals in just two spells with the Portuguese champions. And after bagging another two goals on international duty in the World Cup against Japan, the 27-year-old is quickly making a name for himself in the footballing world.

United have been heavily linked with bringing the Colombian to the Premier League, and could make a move even before van Gaal arrives at Old Trafford after the World Cup. But who else could join Martinez at United? Here are 6 players that may move with the World Cup's hottest property.

[ffc-gallery]Click on Thomas Müller to reveal all

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Thomas Müller

The FC Bayern forward is loving life right now. Despite having to go to extra time to get to the quarter-finals of the World Cup, Müller and his German teammates are their mainly due to his goals. The false number 9 has 4 goals to his name in Brazil and has created a further two. Only James Rodriguez of Columbia has scored more. It means Müller now has 9 World Cup goals in his career, and is expected to surpass the record of 15 World Cup goals scored overall, held by Ronaldo and Müller's international teammate Miroslav Klose.

And he's only 24-years-old. An already experienced striker who is yet to reach his late twenties, Müller is the perfect man for Manchester United. He can play out on the right, centre of midfield and as a striker, making him extremely versatile. We know van Gaal is a fan of him, because he gave the forward his first start for Bayern back in 2009.

Stefan de Vrij

Quite a few of Holland's current World Cup squad will be looking to impress van Gaal so much that the manager ends up signing them for club duty too, and one man in a great position to do so is de Vrij. Just Feyernoord defender is only 22-years-old, yet has started all 4 of Holland's World Cup fixtures so far. de Vrij has looked focused and composed for the Dutch, reading the game well and stepping in to break up the play when needed.

The 22-year-old looks to be a great prospect for the future. And with United needing to replace Ferdinand and Vidic who have departed, van Gaal could bring in the Dutchman, and mould him into a United star. Who knows, de Vrij could arrive at Old Trafford as a World Cup winner.

Nigel de Jong.

He may be a former Manchester City man, but that wouldn't stop de Jong being welcomed to Old Trafford in anticipation. Everybody knows what the 30-year-old brings to the game. A fantastic discipline, a strong challenge and a great eye for intercepting possession, de Jong could boss the Manchester United midfield, and provide the security in the centre of the field that was lacking at times last season. Whilst Carrick does a good job in possession for the Red Devils, he doesn't carry the aggression and tackling ability of de Jong.

De Jong could be for van Gaal what Roy Keane was for Sir Alex Ferguson. Just maybe with a little less controversy. A powerful midfielder, de Jong will grab the match by the scruff of the neck, disrupt opposition attacks and turn defence into offence. Trusted by van Gaal, the former City man could become a hit at Old Trafford.

Thomas Vermaelen.

As van Gaal looks to fill the giant gap in defence at Old Trafford, one man likely to join this summer is Thomas Vermaelen. The Belgian centre-back, who can also cover at left-back if needed, fits van Gaal's desire to have a ball-playing centre-back. Vermaelen is comfortable in possession, not put off by oncoming pressure, and could be the latest player to depart Arsenal for a rival team. After being frozen out of the first team at the Emirates, the Belgian has a point to prove should he move to United.

The 28-year-old is decent in the air, able to find a pass to a midfielder capable of unlocking defences, and not the slowest of centre-backs in the league. An all-round decent purchase for van Gaal should a deal happen once the World Cup finishes, the question will be if the Belgian causes Wenger to regret selling him to United.

Sami Khedira

Another option to strengthen the United midfield is German midfielder Sami Khedira. The 27-year-old, who missed a long spell of football with a knee injury, has a lot of competition at the Bernabeu, and could soon struggle for first-team action with Alonso, Modric, Isco and Illaramendi all battling for a place in the side with the German. But with United interested in making their midfield more secure, Khedira could be offered a place with United.

The Real Madrid man is a well disciplined midfielder, who is rarely interested in going forward and scoring goals. Instead, the German will happily sit and break up the opposition counter attacks, before getting his side back on the front foot. In his prime years and with top experience behind him, Khedira could be a great option for van Gaal to create a dominant central midfield at Old Trafford.

Angel Di Maria

The winger has made it clear that he's not interested in warming the bench at the Bernabeu. The Argentine, who scored the winner against Switzerland last night in the World Cup, has been linked with both United and Arsenal over the past months, and is desperate to prove himself on the big stage. Struggling to fight with Bale and Ronaldo at Madrid, the Champions League winner, who was arguably the best player on the field during the final, is ready to show the world what he's all about.

Real are always going to be linked with exciting attacking midfielders, all of whom are speculated to be in replace of Di Maria. With this in mind, the Argentine seems happy to call the end on his time in the Spanish capital, and United could benefit from that. Quick and full of tricks, Premier League wing-backs will be hoping Di Maria doesn't move to Old Trafford. Red Devil fans would love to see an attacking midfield of Rooney, Mata and the Argentine.