Goals from Ezri Konsa and Boubacar Kamara were enough to secure a crucial three points for Aston Villa, keeping them in the race for Champions League qualification.
Defeat to Unai Emery’s side marks their 21st Premier League loss so far this season as they sit all the way down in 17th.
Postecoglou fielded a much-changed side, keeping one eye on the Europa League final against Man United on Wednesday (May 21st).
Speaking to the press after the game, the Australian was frustrated with his side not being able to keep up with Villa.
He said: “I thought up until they scored the boys had worked hard, they were really disciplined and organised. We didn't really let them create too many clear openings.
“We had our moments as well, going the other way, but once they scored we lost a little bit of belief. We got a little bit fatigued.
“A lot of those guys haven't played a lot and we looked like we were fatiguing and then the game just got away from us.”