The big left-hander is scheduled to start Friday against the Chicago Cubs in Mesa. It would be Johnson's only Phoenix-area start of the exhibition season.
The discomfort surfaced when Johnson worked out on Tuesday at the Diamondbacks' training facility in Tucson.
"Today he was throwing at 8:30 this morning and felt it a little bit so we shut it down," Showalter said.
If Johnson doesn't pitch against the Cubs, he may throw a few pitches in a minor league game in Tucson, Showalter said. A decision is expected by Thursday.
Showalter said the problem isn't serious but there was no sense in taking any chances with the start of the season less than two weeks away.
Johnson is scheduled to make his final exhibition start March 31 against the Colorado Rockies at Tucson Electric Park, then go against Kevin Brown in the season opener at Dodger Stadium on April 5.
Johnson has pitched 13 innings in four starts, going 1-0 with a 3.46 ERA and 16 strikeouts.
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