Louis van Gaal has said he is sick of defending Wayne Rooney this season so no one asked him to after the Capital One Cup exit to Middlesbrough.

There was nothing to say in any case, the story had unfolded on the pitch. Rooney did not start the game. As is now common Capital One practice he was rested on the bench alongside Juan Mata and Anthony Martial, while young guns such as Andreas Pereira and James Wilson were given the chance to show what they could do.

At half-time, with the game goalless, Rooney was sent on because Wilson had picked up an injury. Van Gaal confirmed that was the case afterwards, yet did not choose to explain why it was the captain who had to go out as a replacement when Martial would have been a more natural choice.

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Martial is still only 19, after all, the same age as Wilson. He was signed as a centre-forward, the same position Wilson had been playing, and has been doing well in his first season at the club – he won the Premier League player of the month award for September.

Rooney has not won any player of the month awards this season, or last season for that matter (January 2010, since you ask, between Carlos Tevez and Mark Schwarzer). Apart from finally overhauling Sir Bobby Charlton’s England scoring record this season, the nearest the United captain has come to recognition would be some sort of column inches award for the ongoing debate over his continued usefulness.

Suffice to say, in the interests of brevity as well as diplomacy, he is not enjoying his best run of form at the moment. Indeed he is hardly enjoying any form at all. He probably needs a rest, and the Capital One Cup seems to serve that purpose for senior citizens struggling to cope with two or three games a week, yet it is important to recognise what Van Gaal did on Wednesday evening was subtly different to what Sir Alex Ferguson used to do with his most important players on cup nights.

What Ferguson would do was give the kids a run-out and leave the old pros on the bench, on the understanding that if the game needed chasing or winning the more experienced players would get the call in the later stages. Like a chemist adding strength to a solution with drops from a pipette, as someone once memorably described the process, Ferguson would try to win games while taxing his overall resources as little as possible.

Van Gaal was not quite doing that against Middlesbrough. His withdrawal of Wilson was not tactical but enforced, and he sent on Rooney not to beef up United’s attacking resources – if he hoped that would happen he ended up disappointed – but because he thought the 30-year-old was the best replacement for a centre-forward. Martial, by a mile, would have been the like-for-like replacement, yet when the French teenager finally made the pitch, when Van Gaal really was making substitutions to chase the game, he was sent out to the left wing.

This distils the debate that has been raging in Manchester for the past few weeks. It is not that Martial is a poor option on the left; he can be reasonably effective in that position even if he found himself marginalised during normal time on Wednesday. What most people are having difficulty understanding is why Rooney should be considered a better option in the centre when, to be brutal, his first touch seems to have gone the way of his pace and he tends to slow up every United attack.

There may be a case for playing Rooney as a withdrawn striker or No10 behind Martial, though on the evidence against Boro, not much of one. There is no case at all for playing Martial out wide to accommodate a captain who is clearly struggling, apart from the fact that Memphis Depay has yet to convince anyone he can provide the width and penetration United need.

Depay, in fact, was arguably a bigger flop than Rooney against Championship opposition. Given a chance to shine after a few weeks out of the side, he did not do anything to suggest he deserves a recall this weekend, and was off the field by the time the game went to penalties.

Rooney failing with his side’s first seemed to sum up the whole situation. What should a guy in his position do? He is off form and struggling for touch but on the other hand he is the captain and rightly prepared to accept responsibility. There is no particular blame attached to missing a penalty in a shootout – anyone can do it – though the episode could be read as further evidence that Rooney’s natural game has all but gone. For virtually all of this season he has looked like a player trying too hard.

This is not to say he is finished but he does look like he could use a proper rest. Rooney made absolutely no difference when he came on against Middlesbrough, so in all likelihood it would not hurt United to stand him down against Crystal Palace on Saturday. Van Gaal is going to have to think about the captaincy too. Manuel Pellegrini made the point last week that Rooney could last a few more seasons, though not if he has to play in every game. If the United captain has to play in every game, then that captain should not be Rooney, not in his present form. If Rooney is going to be used more sparingly in order to prolong his career, as seems sensible, then the armband should not stand in the way.