We need balance and Mat Hummels fits the bill

United tried to sign Hummels in 2014 but were rebuffed by Dortmund but the World Cup winning centre-back is out of contract in 2017 and the Bundesliga side could be forced to cash out.

Hummels decided to commit his future to Dortmund on that occasion but has been heavily linked with the Reds in the past and is known to have a good relationship with Louis Van Gaal. 

Following the German's poor campaign in 2014-15, United did not revive their interest in the former Bayern Munich centre-back last year and instead focused their efforts on signing Real Madrid's Sergio Ramos who eventually signed a new contract at the Bernabeu.

Netherlands legend Ruud Gullitt said last year that Van Gaal was 'crazy' about signing Hummels in his first summer in charge at Old Trafford.

Despite the vast amount of money spend on the squad in his 18 months in charge, Van Gaal has not signed an out-and-out centre back since he took over.
Marcos Rojo has spent the majority of his Reds career in central defence and Daley Blind was converted into a centre half in pre-season, however both have performed more assuredly at left-back.

Van Gaal had hoped to develop the Jones-Smalling axis at the back, but Jones' persistent injury have prevented the pair from building a consistent and regular partnership at the back leaving United short of defensive options.
United's defensive showing at Newcastle rang the alarm bells, even though we actually still have the third-best back four record in the Premier League.

Ashley Young, a winger, started at right-back, Matteo Darmian, a right-back, started at left-back and Blind- a good player but not a central defender- started alongside Chris Smalling whom had his poorest game of the season  at St James'.

Van Gaal had been criticised for going into the season with a 23-man squad which included only three recognised centre-halves in Smalling, Phil Jones and Paddy McNair.
We need balance in the side and despite Mike's marked improvement it's been glaringly obvious that he needs a reliable and imposing presence alongside him rather than an out of position stop-gap whom lacks the physical strength and height to make a success of the position. 

Hummels would be the perfect fit: an experienced, consistent, ball playing and physically strong player he would be relatively cheap and would be the perfect foil to cover any mistakes on the rare occasion Smalling has an off day.

Tuesday's attacking performance was our best of the season but we were let down by nervy and naive defending making our need for defensive re-inforcements greater than ever.