Di Maria, 28, said if he had not played in his side's 2-1 win over champions Chile in the Copa America he was afraid "she was going to get mad".
Argentina coach Gerardo Martino said he was unaware of what had happened when picking the Paris St-Germain forward.
"Not all people respond the same in those situations," said Martino.
Di Maria scored the opening goal before setting up Ever Banega for his side's second as Argentina, without injured captain Lionel Messi, won their opening game in the tournament in the United States.
"I am so happy because I was able to score," said the former Manchester United and Real Madrid player.
"It was for my grandmother and my family that are having a difficult time.
"In the morning when I knew what happened I knew that I would score today."
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Friday marks 12 year since 52 people died and hundreds were injured when four suicide bombers attacked the city's transport system in 2005.
Sadiq Khan said Londoners "always pull together" in light of recent attacks in London Bridge, Westminster and Finsbury Park.
A wreath-laying ceremony took place earlier in honour of the victims.
"As well as grieving those we have lost, we also pay tribute to the heroic efforts of our emergency services and transport colleagues on that darkest of days", Mr Khan said
"Our firefighters, paramedics, police and public transport staff fought to save lives in the most horrific and harrowing of circumstances.
"The way our city stood together in the aftermath of the 7/7 attacks inspired the world.
"When Londoners face adversity, we always pull together."
Suicide bombers Mohammed Sidique Khan, 30, Shehzad Tanweer, 22, Hasib Hussain, 18, and Jermaine Lindsay, 19, brought horror to London on July 7 2005.
Travelling from Luton, they took a train to King's Cross in London, hugged and separated to carry out the atrocities.
Within three minutes of 08:50 BST, Tanweer detonated his bomb at Aldgate, Khan set his device off at Edgware Road and Lindsay blew himself up between King's Cross and Russell Square.
Hussain detonated his device on a bus at Tavistock Square at 09:47 BST.
Angel di Maria paid an emotional tribute to his grandmother after scoring for Argentina on the day she died.
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The victims of the 7 July attacks will "never be forgotten", the mayor of London has said.