Royals right-hander Chris Young was one of the first Major League players to be diagnosed with thoracic outlet syndrome in 2013. More recently, the Mets' Matt Harvey and the Twins' Phil Hughes have been diagnosed. Harvey recently had season-ending surgery.
TOS involves a group of disorders that occur when there is compression or irritation of the nerves and/or blood vessels in the lower neck and chest.
"It was something I had pain for upwards of four years," Young said. "It's a relief to figure out what the source of your pain is.
"The second I woke up from surgery, I knew that the pain I had in front of my shoulder was completely gone," Young said. "I knew right then that, likely, that was the cause of the problem."
Picollo said Zimmer had the neurogenic form of TOS, the most common form, and the quickest to recover from.
"He should be fine for Spring Training," Picollo said.
Zimmer, 24, pitched in only three Minor League games this season before going on the disabled list in May because of shoulder fatigue.
Zimmer has had numerous injuries since being drafted, including issues with his elbow, lat, labrum and shoulder.
"We're just glad to finally be able to get a diagnosis," Picollo said. "It has been unsettling for him to be throwing 95 [mph] one day and 88 the next. Now we know why."
When healthy, Picollo has described Zimmer's stuff as arguably "the best in the organization."
Jeffrey Flanagan has covered the Royals since 1991, and for MLB.com since 2015. Follow him on Twitter @FlannyMLB. This story was not subject to the approval of Major League Baseball or its clubs.