Watch: Ex-Chelsea striker Diego Costa scores his first Atletico Madrid goal five minutes into return
Costa has endured a long road back to play for his old club – it's been over seven months since he last played competitively, having made his final appearance for Chelsea in May's 2-1 FA Cup final defeat to Arsenal.
After arriving overweight at Atletico in September following a breakdown in relations with Chelsea manager Antonio Conte, the 29-year-old had to prove his fitness before he was even eligible to play for Diego Simeone's side.
But just five minutes into his return off the bench against third-tier Lleida, Costa was back to his goalscoring ways. Atleti's new No.18 came on for Angel Correa and scored with almost his first touch of the ball in the 69th minute.
With Atleti already 2-0 up thanks to strikes from Diego Godin and Fernando Torres, Juanfran squared a ball across goal before Costa converted with a clever flick.
Antoine Griezmann's stoppage-time effort meant Atletico emerged 4-0 winners and will play their second leg at the Wanda Metropolitano on January 9.