Kuehl, who was best known for co-authoring a pair of baseball books, "The Mental Game of Baseball: A Guide to Peak Performance" and "A Champion's State of Mind", spent 12 years in the A's organization as the director of player development and a special assistant to the general manager.
Byrnes came in contact early in his career with Kuehl and the two hit it off.
"Of anybody, he's someone that had the biggest impact on my career," said Byrnes, who is out for the year with a torn left hamstring.
Byrnes hit a low point in his career in 2005 when he played for three teams and hit less than .200 for two of them. At the end of the year, he found himself without a job and at a crossroads.
It was then that Kuehl reached out to him.
"At the end of 2005 when I had that rough year, he said he wanted to meet with me and we started meeting on a regular basis," Byrnes said. "He had a real stern way of talking and he told me, 'Look, I want to see you to get the most out of your potential and I don't think you're doing so.' I wholeheartedly agreed with him and we mapped out a plan both physically and mentally. He wasn't a psychologist, but he taught an attitude and a way of thinking that has helped me tremendously."
Byrnes signed with the D-backs during the offseason and had the two best years of his career in 2006 and 2007.
Kuehl's death on Aug. 6 hit Byrnes hard and he struggled with whether he should go back for the memorial service with the D-backs involved in a huge three-game series with the second-place Dodgers.
"To leave, especially a series like this, is hard," Byrnes said. "I don't know that I'd do it for anyone else, but nobody had more of an impact on my career than Karl Kuehl."
Byrnes wasn't the only one touched by Kuehl, who was the recipient of the 2006 Roland Hemond Award, which recognizes individuals for their long-term contributions to scouting and player development.
Byrnes is expected to rejoin the team Monday in San Francisco.
Steve Gilbert is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.