Francescoli oversaw River's sale of Mastantuono to Real Madrid this past summer.
The youngster has just made his competitive debut for Argentina and Francescoli told AS: "He's a young man with enormous talent and great maturity. Beyond his excellence on the pitch, his personality is also striking. He's just turned 18, but on the pitch he behaves like someone much older, with more experience.
"His great technique helps him play with ease, but his intelligence and personality are essential qualities that will stay with him throughout his life. He's a player unlike any other. I have no doubt about that.
"There must be a reason why he's so young and new. In elite football, you don't give anything away. He may not be making a difference yet, but he's new and needs to adapt. Time will tell what he's capable of."
Rival offers
Francescoli was also asked if Real Madrid faced competition for Mastantuono's signature.
He said, "Yes. He attracted great enthusiasm from his debut, and there was interest from several clubs, but when Real Madrid arrived, Franco ruled out everything else. He was always clear about his choice, and I think he made the right choice.
"This is his place; he's a special player for a special club, one we admire and with whom we have a wonderful relationship at River Plate. That's why, when this option arose, we all pursued it and, fortunately, we reached a positive agreement in the negotiations."
PSG was also mentioned and he added: "It's true that there were talks, and the young player himself admitted to having spoken with Luis Enrique, but Real Madrid reacted quickly, and that was the end of the French options and those of the other interested parties. We were all determined to reach an agreement through Real Madrid."