The Sounders enjoyed more possession and four more shots on goal than opponents LA Galaxy but were hit by a sucker-punch in the 58th minute when Juninho struck with a brilliant goal which ended up being the winner.
Keller, who could do little to stop the bullet from the Brazilian, said he doesn't understand how the Sounders went down in the season opener.
"Today, there was no reason to lose this game," said Keller.
"It's just one of those things. One goal is the difference. We had the opportunities. On another day, maybe we score three and they score none. ... I don't know if we did enough to comfortably win game. We definitely could have gotten something out of this game."
Seattle coach Sigi Schmid admitted it was a poor result but said it was just a slight aberration at the start of a long season.
"I've said before this game, there's going to be 33 games left after this one and there's still a lot of points out there," he said, "so we've got to lick our wounds, go forward and take care of the points as they go."