Harrison, 27, was 18-11 with a 3.29 ERA in 32 starts last season. That included 213⅓ innings, a career high.
He made his first All-Star squad and is in discussions with the Rangers about a long-term contract extension.
"It was truly an honor to be asked to be a part of a team that represents our country, but at the same time I didn't want to risk injury after throwing a career high in innings," Harrison texted Tuesday.
Harrison said he's focused on getting prepared for the season. That includes working on a slider, a pitch he hasn't used often.
"It's my fourth pitch right now and I want to get that better," Harrison said Friday at the Texas Rangers Fan Fest in Arlington. "I need to use my off-speed more to the arm side and make it a backdoor pitch to righties. It wasn't there consistently enough and I want to work on that this offseason and at spring training."