The 22-year-old winger joins Villa on an initial loan deal with an obligation to buy, and says that it was Emery that made him want to make the switch to Villa Park.
"I think I was looking for a club that I could get confidence and help me try to be the player I was years ago from my first years at (Manchester United)", he told Chelsea's club website.
Garnacho made a £40 million pound move from United to Chelsea last summer, but has failed to impress at Stamford Bridge.
In 24 appearances last season, the Argentinian scored just once while providing two assists.
He continued: "I spoke with the manager and he's given me that confidence.
"I saw them in the Europa League last season and to be a part of the Champions League is important and with Unai, I spoke with him before I came here and that's what's given me the confidence."