Rojo was rushed to San Martin Hospital with a bullet wound to his shoulder but lost his fight for life some seven hours later.
Respected La Plata newspaper Hoy, which identified the younger victim as Marcos Rojo’s cousin, published distressing pictures of the teenager and the dead man lying on the ground next to their motorbike.
The man thought to be the older of the two was lying face down in a pool of blood.
The one believed to be the footballer’s relative was on his back with blood appearing to be streaming from an injury to his left shoulder and his face partially hidden under a baseball cap.
Police sources told local press the pair had tried to rob a taxi driver who had managed to get away by accelerating before they targeted the ex-policeman.
The violent shooting is being probed by state prosecutor Betina Lacki.
The retired police officer is said to have told investigators after being arrested as part of the ongoing investigation that the men he shot pointed guns at him and he reacted in self-defence with a.40 calibre pistol.
Rojo, 26, is back in favour at Man United and scored a rare goal at the weekend in his club’s draw against Bournemouth.
His form for his side has earned him a recall to Argentina’s 26-man squad for World Cup qualifiers against Chile and Bolivia later this month after he was dropped last November.
He has played regularly for Man United after being drafted into the centre of defence last October.
Rojo has had his own brushes with the law in his homeland.
He was formally accused over a November 2010 alleged bottle attack on a neighbour in November 2012 and warned he could be handed a one-year jail sentence if convicted of wounding.
He lifted the threat of prison hanging over his head by striking an out-of-court settlement with his accusers, leading to the case against him being discontinued at the start of 2015.
Neighbour Juan Pablo Gomez had claimed the footballer smashed him over the head with a bottle, injuring him in the face, body and forearm, before assaulting Juan Pedro Celiz as he rushed to his aid.
The men were involved in an altercation with Rojo and the Man Utd star’s family and friends as they celebrated his move to Spartak Moscow from Argentinian side Estudiantes.
Two witnesses reportedly gave statements pointing the finger at Rojo.
However, police sources quoted at the time claimed the footballer had reacted to a prior assault by neighbours during a violent row involving several people including the Man U’s star dad and uncle.
The threat of trial – and a prison sentence – was still hanging over the defender’s head when he signed for Man U from Sporting Lisbon in August 2015 for £16million.