MESA, Ariz. (AP) -- The Cubs have gone from worrying about Matt Garza's elbow to concern over his side.
Chicago's starter will undergo an MRI to determine the severity of the side injury he sustained while pitching batting practice, his first extensive work since last July.
Garza, who made just 18 starts last season because of an elbow injury, felt a twinge in his side while throwing on Sunday.
The Cubs initially called Garza's injury a "mild strain" but general manager Jed Hoyer said Monday that the right-hander will undergo further tests so team doctors will have a better sense of how long Garza will be sidelined.
"The good news is his arm felt really strong," Hoyer said. "It's unfortunate and it's going to set him back a little bit. But we're still really confident. Our concern was with the elbow injury that shut him down last year."
Garza went 5-7 with a 3.91 ERA last season. He made his final start on July 21, when he left a game in St. Louis because of a "stress reaction" in his elbow.
Until the Cubs get the test results, it's not known how long Garza will be sidelined. He remains a candidate to be Chicago's opening-day starter, but manager Dale Sveum said his rotation doesn't have to be set until March 20.
"You can't do anything earlier than that just because unfortunate things like yesterday might happen," he said. "By then (March 20), everybody needs to know what's going on and what their roles are going to be."
Jeff Samardzija and Edwin Jackson are the other possible candidates to start the April 1 opener at Pittsburgh.