The loss brought to an end a run of six games unbeaten for Saints, while Fraser Forster was finally beaten after 708 minutes without his goal being breached.
Saints were leading at the break through Shane Long, but Koeman admitted his side’s lack of possession was their downfall in the second 45 minutes.
“I thought we played well in the first half and made the one-nil really at a good moment in the first half before half time,” Koeman told youtube.com/southamptonfc
“We had good defending but in the second half we had more problems to keep the ball and it was more defending than in the first. Still we were good defensively.
“They didn’t create open chances, but we had to drop too much back. Maybe the first goal was an unlucky situation because it’s not really a shot.
“It’s a cross and nobody touches the ball. Then it’s one-one. It came out of a situation that the linesman had his flag to tell the referee about a foul, but the referee took responsibility.
“We dropped a little back more than we like to. That’s also about keeping the ball in front so the defenders get the time to push up.
“If you lose more balls, then it’s more difficult.”
Cesc Fàbregas’s goal in the 75th minute ended Forster’s run of 708 minutes without conceding and despite his goalkeeper becoming a Saints record holder, Koeman admitted it was a difficult moment to enjoy after defeat.
“It’s maybe nice in 20 years if you keep that record,” Koeman added. “It’s nice for Fraser, but finally we lose the game and that’s the disappointment we have.”