He was promising back then in Monaco, but Ranieri decided to send him away in order to get more play time. He played realy well for NIce this season.
To compare with Kante, he isnt realy the same type. Just because he arrived, it doesnt necessary mean Kante will leave. Kante is b2b, Mendy is more technical Makelele. Parked infront of defence, distributing balls. His passing is realy good and has nice ball control. I think under Ranieri, he could be one of players to watch next season because his potential is even higher than Kante's.