Di Maria returned to Argentina this summer to Rosario Central, while Mastantuono left River Plate for Real Madrid.
"He's had a fantastic start in the Champions League, hey? I think he has a lot more quality than I ever had, haha. Honestly, I'm really happy for him," Di Maria, 37, tells DSports.
"The way he connects with his teammates so quickly, his skill, that bold attitude of always wanting to take on opponents and push forward - it's something really special. People will definitely appreciate it."
The former Real Madrid star is particularly impressed given Mastantuono's age.
"For someone who is only 18 years old, what he is doing shows incredible character," says Di María.
"I'm really happy that an Argentine is once again representing Real Madrid and doing as well as he is."