Prior to the Giants' game against the Pittsburgh Pirates, Johnson was transferred from the 15-day to the 60-day disabled list to clear room for newly acquired Ryan Garko on the 40-man roster. That meant the Big Unit can be activated no earlier than Sept. 4, when his team could be fighting for a postseason berth.
Johnson said an MRI performed Monday by team orthopedist Ken Akizuki on his strained throwing shoulder revealed tearing in his rotator cuff. The 45-year-old vet declined to speculate if it would be more realistic to attempt a comeback as a relief pitcher, given he would require time to regain his arm strength.
"I've overcome a lot of surgeries. ... This is basically one more speed bump in the road of my career," he said.
Johnson will not attempt to throw the baseball for at least three more weeks and will continue to rehab the injury. The Giants originally placed him on the 15-day DL on July 6. In the three weeks since, inflammation and swelling in the area has subsided.
"We're still optimistic that we can get him back on the mound [this season]," manager Bruce Bochy said. "We were hoping to get Randy back a little bit sooner, but, you know what, you have to focus forward."
Johnson's absence has pushed Barry Zito into the No. 2 slot of the Giants rotation and will continue to give Ryan Sadowski job security as the team's fifth starter. The rotation -- which still leads the National League in ERA (3.57) -- will also continue to rely on Cy Young Award candidates Tim Lincecum and Matt Cain, as well continued improvement from Jonathan Sanchez.
"Winning 300 games is great, and I knew, obviously, if I was healthy, that was going to be achieved," said Johnson, who signed a one-year, $8 million free-agent contract with a Giants in the offseason. "But I came here to win a lot more games and help this team in some capacity to get this organization to the next level. The team is doing fine in the rotation and [with] the latest acquisition, with Ryan being here."
Johnson said he's plenty motivated by the Giants' playoff chances and that rehabbing his shoulder offers him the best chance to return to action. He added that surgery isn't a likely option.
Prior to Tuesday's announcement, Johnson had been working out in Phoenix six days a week with Brett Fischer -- the same trainer who helped the pitcher rehab following his three back surgeries and two knee surgeries. Johnson said his goal is to build up the small muscles around the affected area, and he'll do that in the Bay Area while the Giants are home and in Arizona when the team takes to the road.
Before the results of his MRI were available, Johnson told reporters Monday, "There's times when it aches a little bit, but I feel relatively good. What does that mean? I don't know. Because I haven't picked up a ball since the last pitch I threw against the Houston Astros [on July 5]."
Andrew Pentis is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.