Formerly a highly touted prospect in the Angels system, the left-handed-hitting McPherson is vying for the Marlins' starting third-base job this spring.
He is competing with Jose Castillo and Jorge Cantu for the position vacated after Miguel Cabrera was traded in December to Detroit.
"Right now we're holding him back a bit," Marlins manager Fredi Gonzalez said. "We're being precautionary."
The team is hopeful McPherson will be ready for his Grapefruit League game on Monday, when the defending World Series champion Red Sox visit Roger Dean Stadium in Jupiter, Fla.
Gonzalez didn't know which side of McPherson is feeling the discomfort, and it is unclear if it is oblique related.
The team says that the setback is not back related. McPherson missed all of 2007 recovering from back surgery.
While McPherson is not yet ready for games, he is hitting off a tee and doing soft-toss hitting drills in the batting cage.
The Marlins have options on McPherson, Castillo and Cantu. While it is unlikely all three will make the team, it is a possibility. And whoever doesn't make the final cut could be optioned to Triple-A.
Joe Frisaro is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.