Di Maria played a key role in Madrid's successful Champions League campaign last term, yet appears to be surplus to requirements following the arrival of James Rodriguez and Toni Kroos this summer.
Madrid coach Carlo Ancelotti revealed earlier this week that the former Benfica star has requested a transfer after turning down an offer to sign a contract extension at the Santiago Bernabeu and the 26-year-old is widely expected to leave the club before the end of the month, much to the surprise of Menotti.
"I think Di Maria should be considered as the best player in Spain over the past few months, outshining even Messi," the 1978 World Cup winner wrote in his column for DPA Global Media Services.
"The way Madrid have treated Di Maria is an injustice.
"They signed James Rodriguez in an attempt to sell more jerseys. He's a marketing product for Madrid."
Di Maria joined Madrid from Benfica in the summer of 2010 and has since made over 100 appearances in La Liga for the Spanish giants.
He has a contract at the Bernabeu until June 2018, but could be available for a transfer fee in the region of €60 million.
The attacker has been heavily linked with both Paris Saint-Germain and Manchester United, but the Ligue 1 champions recently announced that Di Maria is too expensive for them, seemingly leaving Louis van Gaal's side in pole position to sign the versatile attacker.