"The good news is his arm felt really strong and he was throwing really well," Cubs general manager Jed Hoyer said Monday. "It's unfortunate for sure, and it's going to set him back a little bit, but we're still really confident."
There is no timetable for when Garza can resume throwing.
"At this point, it's a little bit of a wait and see," he said.
Garza was expected to have the MRI within the next 24 to 48 hours.
Hoyer was behind the batting cage when Garza was pitching. The right-hander stepped off the mound and started stretching.
"It wasn't the most fun live BP session to watch," Hoyer said. "He looked great and free and easy where you feel really good about what you're watching, and the next thing you know, you see a guy stretching his side.
"Right away, when you see a guy like that stretching, you think the worst. It was a bummer, but I'm glad his arm feels good, and Matt's in real good spirits because of that. It's the kind of injury that you're thankful happened in the first live BP. When these injuries happen in the last start of Spring Training, it really hurts you in the season."
A strained lat can be a lingering injury. Hoyer said the Cubs hope it's a mild strain and nothing more.
Garza, who was 5-7 with a 3.91 ERA in 18 starts last season, was one of the candidates to start Opening Day, but Cubs manager Dale Sveum said that decision doesn't have to be made until March 20.
"You can't do anything earlier than that just because unfortunate things like yesterday might happen, and then you'd have to regroup to do different things to get guys set up for the first day, third day of the season," Sveum said. "By [March 20], everybody needs to know what's going on and what their roles are going to be and when they're going to pitch."
Jeff Samardzija and Edwin Jackson are the other candidates for the Cubs' Opening Day assignment. Samardzija avoided injury in his live BP session on Monday, when he danced off the mound to avoid being hit by a comebacker from Darwin Barney.
"We don't need that," Barney said.