The pair fell out when working together at PSG.
But Emery says: "He's a very good player and he's playing with big performances.
"We know they have good organisation and good players - and one is Ben Arfa.
"We need to control his quality and his skills because in the two matches they played against Betis he played very well with big performances.
"But it's not only Ben Arfa, they have Sarr, Grenier, Bourigeaud, Andre and some good defensive players. It's all the players. But mostly, Ben Arfa is very important for them
"They played very well again Real Betis in the two games [in the last-32], and they deserve to be here. They have very good organisation defensively and offensively. They have very good players and we know for them it's an important moment, and for us it's the same.
"We'll prepare for the match thinking the best of them, wanting to stop their good players, but to impose our gameplan with our capacity and quality. We know we need to play two matches with big performances [to go through]."