Koeman commended the defensive work of his players as crucial in achieving a 2-0 win over Sevilla at the Estadio Ramon Sanchez Pizjuan.
The visitors started with three centre-backs and two wing-backs in Nervion - a formation they stuck with for the whole 90 minutes.
"We tried to press our opponent but playing with three centre-backs and high wing-backs," Koeman said at full time.
"It made it easier for us to stop Sevilla playing. That was the key.
"It was a risky system. Without the ball, we pressed, we tried to play man to man.
"Our game without the ball was key. It was a very complete match."