"We evaluated him," said head athletic trainer Barry Weinberg. "Dr. [Lyndon] Gross evaluated him and thought he had some tightness and some weakness in his left shoulder. So we will have an MRI done on Monday, [then we'll] re-examine and evaluate. He won't throw till at least sometime after we get the results."
Johnson began to warm up during the eighth inning of the Cardinals' game against Florida. But according to pitching coach Dave Duncan, the lefty told bullpen coach Marty Mason that he didn't feel right. Mason had Johnson cut his warmup short, and Johnson did not appear in the game.
Johnson chose not to comment on the situation following the game.
"He can't externally rotate the same as he did at the physical," Weinberg said. "That's why we take baseline studies. He also had weakness in manual muscle tests.
"Any time you stop a guy from throwing, it's a concern. Now we just want to make sure that there's no significant issue."
Johnson, 26, has not appeared in a Grapefruit League game this year. He posted a 4.03 ERA with 24 strikeouts and 16 walks over 55 games (38 innings) in 2007.
Matthew Leach is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.