The Belgium midfielder is in his second season as a Villan.
Ahead of tonight's trip to Manchester City, Emery said: "We followed him even before I arrived here.
“He was playing at Anderlecht, Monaco, Leicester. And when I was in France, in Paris, he was playing in Monaco.
"I always appreciated him a lot as a player for my idea, my style to set with the players we have. He was one player, sometimes, for one thing like Aston Villa. We were two or three years ago, not easy to sign him.
"We tried, in that moment we tried to convince him. There was a moment he was not performing in the level he usually had before. For different circumstances.
"Leicester, they were relegated. They were not performing relatively well and he was not performing well.
"Even when we went to watch one match, Leicester versus Everton, they lost at home. In that moment I think they were relegated and he didn't perform well.
"But we were trying to identify in our structure how he can help us to use his qualities and skills and his mentality to help us and to be with us and we signed him.”
First season was difficult
Emery also said: "Of course last year was difficult for him because he needed some time to adapt to us. He had some players in front of him playing, like for example Douglas Luiz.
"But he always showed us is mentality, he showed us his skills. His versatility as well is very important, playing as a number six, eight or a 10. Sometimes even last year he played on the left.
"This year he's been consistent, really consistent. I think he's one of the leaders on the field because he's linking a lot on the field with our style through keeping ball possession and as well defensively.
"His commitment defensively to do his task and how he can help other players to settle as better as possible is his best characteristic as a player. Overall he's a very important player for us, very important."