Despite being 15 points ahead of city rivals Manchester City, there is still evidence that Manchester United has lots of room for improvement.

This upcoming summer transfer window could be crucial for Sir Alex Ferguson to put together a team for the next three years onwards.

Manchester United has been solely the most consistent team in the Barclays Premier League this season, and seems likely to lift their 20th league title. Their success this season has been largely down to their devastating attacking options including the likes of Robin van Persie, Wayne Rooney and Javier Hernandez. 

Despite being dominant in the league, United have failed in the Champions League and got knocked out of the FA Cup at the hands of Chelsea. This will not sit well with Ferguson as he was setting his sights on a second treble winning season under his asministration. 

This transfer window could prove to be one in which Sir Alex captures some great talent to improve the areas that Manchester United needs to strengthen.

Here are five players who could take them to another level and capture the treble next season.

Alex Sandro

Manchester United has always had some very solid left backs which include the likes of Dennis Irwin, Gabriel Heinze and Patrice Evra. Evra has had a good season especially in front of goal, but he has not hit the form that he once showed. For the past two or three seasons his form has dropped significantly and has no been good enough.

Alex Sandro has been in great form for FC Porto since his move from Brazil. He has impressed especially in the Champions League this season and has already been handed six caps for Brazil.

At the age of just 22 he has great energy and is excellent going forward and solid in defence. He certainly has lots of talent and there is no doubt that Ferguson is very capable of allowing Sandro to fulfill his potential.

Alex Sandro was in the Brazil team that won the silver medal at the Olympics in 2012 and with Rafael at Old Trafford, he could be eased into life in Manchester by his fellow countryman.

Kevin Strootman

The young dutch midfielder has been in excellent form since he joined PSV Eindhoven from Utretch in July 2011. Strootman is being dubbed as the 'Dutch Roy Keane' because of his dominant presence and display in midfield for club and country. He is a fixture in the Dutch national team and has captained the team in the absence of Wesley Sneijder at the age of just 23.

Strootman is known for his steel in the middle of the park, great distribution, aerial strength and a good burst of pace. At 6ft 1in, he has the physical presence needed in the Premier League and will be a threat from set pieces.

He is the type of player that the Red Devils faithful has been crying out for since the departure of Roy Keane in 2005 and this summer he could join fellow Dutch players Robin van Persie and Alex Buttner at Old Trafford.

Angel Di Maria

The 25-year-old Argentine has really set the Santiago Bernabeau alight since his move from Benfica in the summer of 2010. He is a player with lots of skills, tricks and pace. He has the natural ability to beat his opponents and on his day is virtually unstoppable.

Di Maria is a modern day winger, which means his presence provides a goal threat and has an excellent final ball into the penalty area or in behind the defence.

This season the wingers at Manchester United has failed to be the danger they were in past seasons and therefore Sir Alex may well look to the Argentine to add a new dimension to his attack.

The future of Nani and Antonio Valencia is in huge doubt and therefore it is very likely that Ferguson may turn to the Argentine whose relationship with Jose Mourinho has broken down, and may lead to Di Maria's exit from Real Madrid.

Robert Lewandowski

The Polish international has been a revelation in the German Bundesliga for Borussia Dortmund since he arrived at Signal Iduna Park.

Lewandowski has help Dortmund to back-to-back titles and the German Cup. This season he has been in great goal-scoring form for the BVB and has help them reach the quarter finals of the Champions League.

He has scored 20 goals in the Bundesliga so far along with five assists and he has five goals in the Champions League as well as two assists. Lewandowski has been linked with a move to Old Trafford since last summer, and could link up with old team-mate Shinji Kagawa this summer.

He can certainly join the wealth of strikers at Old Trafford as he is very strong in the air, is great at holding up the ball and his link up play is superb. His addition would mean more goals for Manchester United and could help them to go all the way in all competitions.

Mats Hummels

For the past two season Mats Hummes has arguably been the best defender in Europe. He is a ball-playing central defender who is very strong in the air, great in one-on-one situations and reads the game very well. He also has a great distribution from defence and is a threat from set pieces as he has scored 17 goals so far in almost five seasons for Dortmund.

Hummels is also a fixture in the German national team and featured in Euro 2012 leading Germany to the semi-final before being eliminated by Italy.

With Rio Ferdinand coming to the end of his career and the injury problems of captain Nemanja Vidic, Hummels would be a great adddition to anchor the Manchester United defence in the years to come.

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#Football
#Premier League
#Manchester United