The Black Cats boss saw the Liverpool striker in his formative years and admitted that he didn't think his compatriot would become one of the top centre-forwards in world football.
The Mirror quotes him as saying: "I went to a friendly game. The big teams play against each other in the summer, and it was a game between Penarol and Nacional.
"Suarez played all right, but not for one minute did I think he would be this good. He was a power striker, playing up front on his own but nothing like the striker we see today.
"I doubted him, but I was wrong! Now people ask: is he the best Uruguay has produced? I say yes, why not? When he came to England and started playing I thought 'Mama mia what a player.'"
Meanwhile, Suarez has claimed that he will be fit for their meeting with England on Thursday, despite undergoing knee surgery at the end of last month.