Sono coached Mntambo for four years before selling the player to Chippa United this month along with Mxolisi Macuphu.
The former Cosmos skipper scored a brace on his Chilli Boys debut against AmaZulu and put in another man-of-the-match display in the goalless draw with Golden Arrows last weekend.
"Mntambo is a different player. For my money he's the best midfield player here in South Africa, don't ask me in the NFD or Premier [Soccer] League. He's a box-to-box midfield player. I've never seen a player like this, " said Sono.
"When he came to us he was not doing well and playing street football, but we changed him. You know he has been with me for four years.
"It took him two years for people to take notice that 'this is player'. Two years being with me and unnoticed, and he's become a hell of a player. I've never seen a player that doesn't stop on the field. He's always busy and he's dangerous. We taught him to get in the box and now he knows how to get into the box. Unbelievable player," concluded the legend.