The left-hander has been sidelined with tendinitis and a rotator cuff strain in his left shoulder. He won't throw a baseball at all until the middle of next week, at which point he may begin building back up again toward game activity.
"We're talking at least two weeks from now before he's up on a mound again," said Dr. George Paletta. "Two weeks from now gets us to [March] 19 before he's even up on the mound. He's going to have to throw a couple [of bullpen sessions]. I think there's no question it jeopardizes him for Opening Day."
Paletta explained that Johnson's condition is similar to maladies he has dealt with in the past.
"Tyler's shoulder is one that has a tendency to get tight. ... When he is diligent with his stretching and with his rotator cuff program and with his maintenance work, the likelihood of him developing a problem is much smaller," he said.
"This is in general -- not absolutely, but in general -- a treatable and somewhat preventable issue by doing that work that needs to be done."
When the initial diagnosis was reached on Tuesday, Johnson expressed great relief that he would not require surgery. Paletta emphatically reinforced that notion.
"We sent his MRIs out to Dr. [Lewis] Yocum, who reviewed them," he said. "Yocum called me this morning, and his quote was, 'What's the big deal? He's got a throwing shoulder with a little bit of a strain in the rotator cuff. His labrum looks absolutely perfect.' And Yocum said, 'I wouldn't operate on him unless you want to be the one to end his career.' "
Johnson's absence would have a couple of effects on the Cardinals' bullpen. First, it would improve Ron Villone's chances of making the team as a non-roster invitee to camp. Moreover, it would remove Villone from the list of candidates to serve as a long reliever and open a new competition for that position. Assuming Johnson is out, Villone now projects as more of a specialist in the early part of the season.
"It gives [Villone] a [great] opportunity," manager Tony La Russa said. "From what we've seen with Villone, we have a real chance there. Based on what we've seen, I think Villone can do it. They've used him [in other ways] because he can get right-handers out, but I think he can be really tough on left-handers."
Matthew Leach is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.