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.
'I couldn't ask for more from my second debut.
Vietto is under contract with Atletico until June 2021, but a good spell at the Mestalla outfit could trigger an anticipated exit for a player that was never able to respond to the expectations around him.
Costa started the move that led to his goal, driving forward before moving the ball wide, allowing Juanfran to whip in a low cross that Costa clipped past Lleida keeper David Oliveros.
The Brazilian-born hitman looked to have injured himself in scoring the goal, but managed to finish the game.
"It is great for me to play for Atletico Madrid again, the club are very important to me and I am very happy with this".
DIEGO COSTA marked his return to Atletico by scoring just five minutes after climbing off the bench. I just want to get in shape to play as soon as possible, I'm exhausted of training.
Atletico Madrid is now second in La Liga but nine points behind Barcelona, and the club has scored just 25 goals in its league campaign, which ranks just 39th across Europe's big five leagues.