Manchester United's full set of defensive options inspire little confidence at the best of times. Withdraw the four centre halves who were tasked with filling the void left by Rio Ferdinand and Nemanja Vidic and the problem plunges to new depths.

The sight of the patchwork back line consisting of Antonio Valencia, Paddy McNair, Michael Carrick and Luke Shaw finishing the game against Manchester City once again threw the club's summer transfer decisions under the microscope.

While the foursome coped admirably against Manuel Pellegrini's Manchester City attack, given the circumstances, the increasingly familiar sight of defenders dropping like flies continues to undermine United's progress.

And the cupboard at Old Trafford is barer than it ever has been.

A case for the defence?

The initial loss of Phil Jones, Chris Smalling and Jonny Evans in October saw Louis van Gaal turn to McNair, whose mature performances after being thrust into the first team has vindicated the manager's decision. The 19-year-old's involvement is likely to continue given the latest set-back that sees Marcos Rojo sidelined with a dislocated shoulder.

While the first-team quartet recover from injury and serve bans, Van Gaal is likely to deploy Michael Carrick at the back. Tyler Blackett represents another option but the Dutchman could do with dipping into the Under-21 side to fill out his squad.

Marcos Rojo
Marcos Rojo has brushed off the shoulder injury he suffered against Manchester City Getty

Under-21 captain Tom Thorpe would appear to be the logical short-term solution until at least Chris Smalling's return, providing his manager has no further punishments in mind for the latter after his absurd decisions at the Etihad Stadium.

Thorpe, also an England Under-21 international, has captained Warren Joyce's Under-19 side for two seasons and is the last man standing at that level from the side that won the FA Youth Cup alongside Paul Pogba, Ravel Morrison, Ryan Tunnicliffe, Jesse Lingard and Will Keane in 2011.

His commanding presence at the heart of defence, combined with the leadership qualities that have seen him wear the captain's armband for both club and country, suggest he is of the right character to grasp the challenges Van Gaal has already passed to his teammates.

But when it came to handing out the chances last time United's defensive options were this scarce, Van Gaal opted for the younger McNair, handing the Northern Irishman his full debut in the 2-1 win over West Ham United last month.

While Thorpe also made his full debut in that game off the bench, it was in the dying moments of stoppage time as United held on for a narrow 2-1 win. When it came to Van Gaal selecting a right-footed centre-half to offset his absences in the Manchester derby on Sunday 2 November, it was again McNair who received the shout.

Will Tom Thorpe stay or will he go?

Van Gaal's history of overlooking Thorpe illustrates a worrying precedent for the defender. The youngster was left out of the squad that included the likes of Reece James and Blackett that travelled on the pre-season tour of the US.

He also failed to make the starting XI in the largely experimental side that lost 4-0 to MK Dons in the League Cup, where full-back Marnick Vermijl was handed one of the three central defence roles ahead of the Under-21 skipper, despite never having occupied a central role before.

While McNair has justified his inclusion, it seems the United boss is reluctant to hand Thorpe a similar opportunity. What role, if any, he has in the first team in the coming weeks is likely to determine whether the 21-year-old has a future at the club.

Given the paucity of options available to Van Gaal for the visit of Crystal Palace on Saturday 8 November, Thorpe's full first-team audition needs to come soon if it is to come at all.