Le Havre sold the Algerian to Leicester for £400,000 in January 2014.
“I was confident he would be a success, but I hadn't imagined that he would become such a major player in the Premier League so quickly," Mombaerts told the Sunday Mirror.
“Throughout his first season with Le Havre I had to fight with the leaders of the club just to justify playing him in the first team.
“A lot of people were sceptical about his qualities and didn't believe in him. I'm not sure that too many other clubs in France would have taken a chance on him but we built on the qualities that he clearly had.
“Riyad was a more complete player when he left Le Havre than when we first had him – he had made huge strides in his defensive play and was physically far more ready for the English league."