The Galaxy spurned a number of scoring opportunities at Dick's Sporting Goods Park to eventually go down to Vicente Sanchez's 20th-minute effort for the Rapids.
It was a performance of uncustomary inefficiency from the Galaxy, which was touched on by Keane, who felt that the 2011 and 2012 champions were well below their best.
"We certainly didn't deserve the ending of the game, I don't think. I think that's probably one of our worst performances certainly for a long time," said Keane.
"We looked very sluggish and didn't look like we could get the ball down and pass.
"Sometimes you can have possession just for the sake of having it, so there was no penetration. We just looked very, kind of lackadaisical. It was a poor performance and probably one of the worst since I've been here anyway."
Keane, who has scored four goals this season, also suggested that he was simply not good enough as the Galaxy fell to sixth on the Western Conference table.
"For me, myself, I wasn't good enough and I'll take responsibility for that. I take it on the chin, but there are 11 guys that could say the same thing," the former Tottenham ace added.
"It wasn't good enough, but it's about how we bounce back. There's no point in crawling under a tree and hiding, but heads up and get on with it on Monday."
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