Manchester United down the years have prided themselves on producing world class centre-back partnerships.

We can think back to Martin Buchan and Gordon McQueen in the late 70's and early 80's, Steve Bruce and Gary Pallister in the early 90’s - securing United’s first title for 26 years - Jaap Stam and Ronny Johnsen in the 1999 treble-winning team, and then Rio Ferdinand and Nemanja Vidic, who won a combined 11 Premier League titles.

Unfortunately, during the 2014/15 season it was impossible to recognise the potential of a new centre-back pairing being established, and because of this United have been linked with a whole host of defenders this summer. These are the five most likely transfer targets.

Mats Hummels

Mats Hummels picture from Daily Mail
Mats Hummels picture from Daily Mail

Mats Hummels is the current captain of Borussia Dortmund and a mainstay in the German national team’s defense. Standing at 6ft 3in with over 200 games played for Dortmund, he has been capped 39 times for Germany, won two Bundesliga titles, and earned a 2014 World Cup winners' medal, Hummels has size, strength, athleticism and composure on the ball - all the characteristics of a world-class centre-back.

He is already considered amongst pundits, ex-players and coaches as one of the best defenders in Europe and at 26 years old, he is only just entering into his prime. He would be a very welcome and experienced addition to United’s backline.

Sergio Ramos

Sergio Ramos picture from thehardtackle.com

Ramos is the vice-captain of Real Madrid, making over 300 appearances, and in 2013 the Spaniard became the youngest ever European player to reach 100 caps at 26 years and 357 days. He has won pretty much everything there is to win in football, including two European Championships, a World Cup and the Champions League.

At 29 years old, some may feel Ramos is in the latter part of his prime, however his statistics for the 2014/15 season tell a different story. He won 82% of his tackles in La Liga and 88% in the Champions League. He excels in the air and is a threat in the opposition’s box for set pieces. Carlo Ancelotti once compared him to the great Paolo Maldini, due to his leadership, athletic and technical qualities. Manchester United are desperate for this type of win-at-all-costs leader at the back.

Jerome Boateng

Jerome Boateng picture from skysports

The Bayern Munich and German international is known for being a tough tackler as well as a versatile defender who can play across the whole back four. He is the same age as Hummels (26) and has won a similar number of honours, including the 2014 World Cup. He has played over a 100 times for Bayern and has over 50 caps for Germany.

In contrast to Ramos, Boateng won 87% of his tackles in the Bundesliga last year however he only won 63% in the Champions League.

There is perhaps two reasons United fans might be hesitant to see him in the red shirt next season. The first one being the fact that he played for Man City and the second being the image of him falling over while Leo Messi danced around him to score for Barcelona in the Champions League semi-final.

Nicolás Otamendi

The Argentine is currently plying his trade in Spain with Valencia, having transferred from Porto in 2014 after winning three Primeira Liga’s and the UEFA Europa League. He has played over a 100 times for Porto and Valencia combined and has over 20 caps for Argentina.

Otamendi was a stalwart of the 2014/15 Valencia squad which qualified for next season's Champions League and he was also the only Valencia player named in the La Liga Team of the Year.

He is a natural leader, who reads the game very well (he made 111 interceptions last season). He is not scared to make a tackle or put his head in where it hurts and is also a threat offensively similar to Ramos. Otamendi scored six goals last season, while United’s central defenders managed just five. He has a good relationship with compatriot Marcos Rojo so could United fans be looking at an all Argentine centre-back pairing?

Ron Vlaar

Ron Vlaar picture from daily mail

Ron Vlaar is the oldest of the five defenders at 30 years of age and he is definitely the most inexperienced in regards to playing at the highest level. However, there are some plus points that might just push Vlaar further up Louis van Gaal’s summer shopping list.

Firstly, Van Gaal coached Vlaar when he was only 21 playing at AZ in the Eredivisie and they were reunited again during the 2014 World Cup, where the Dutch team finished third. Vlaar played in all seven matches and is regularly acknowledge as being one of the best defenders at the World Cup.

Secondly, on Wednesday 1st July, Vlaar’s Aston Villa contract expires leaving him as a free agent. This means that Manchester United could sign an experienced tough tackling Premier League centre-back at no cost. This transfer would be more of a short term solution but one that might work.

Whoever Manchester United end up buying this summer, the overall consensus is that the fans would like to see at least one of the above defenders joining to enable them to mount a serious title challenge.