MESA, Ariz. -- Chicago Cubs pitcher Matt Garza will begin the 2013 season on the disabled list after experiencing recurring pain in his left side, and Jeff Samardzija will start on Opening Day, the team announced Sunday.

Garza felt a twinge in the injured area after trying to step up his soft-toss rehab program Saturday. The right-hander initially suffered the injury on Feb. 17 during the team's first week of spring training.

"Now we know it is just time," Garza said. "We pushed it and pushed it hard. Everything felt great. Now it is the body saying, 'Let's take a little more time.'

"It happened late [while side throwing on Saturday]. It is not bad, I just think this came out from not pitching seven months. It didn't hurt, [the discomfort] is just not gone. With stuff like this it has to be gone."

Cubs manager Dale Sveum hopes Garza will return to the rotation sometime in April.

"Hopefully this is not a real bad [side strain] where you can't sneeze or laugh," Sveum said. "I don't think it is that bad, but he is just not ready to throw a baseball yet. [The injury] didn't get worse; he is just not ready to step up any extra intensity."

The Cubs start the season on the road April 1 against the Pittsburgh Pirates.

Last season Samardzija was the team's most consistent starter, posting a 9-13 record and a 3.81 ERA.

"What he did his last eight starts, he is a guy you want out there," Sveum said. "He is a guy players rally around because of his work ethic. He has that bulldog mentality. This was a very obvious choice."

Samardzija made the transition from the Cubs' bullpen to the rotation in 2012. He began his training period in November 2011 when most players begin to rest after eight months of baseball. He repeated that offseason regiment in 2012.

"I am excited," Samardzija said. "The best thing as an athlete is to work hard and see positive gains from it. That [helps] you continue to work hard and keep improving. You get a little taste of success and you want to keep it going."

"He is just that guy that gives you that 'game on' when he is pitching that day," Sveum added. " The other four days he is not pitching, he just doesn't sit around. He is into every game. He is always picking his teammates up. He has fallen into that leadership role as well."

The Cubs will have two-fifths of their starting rotation begin the season on the DL. Right-hander Scott Baker, recovering from elbow surgery, is not expected to be ready until mid-April. Left-hander Travis Wood and right-hander Carlos Villanueva will be penciled in as the fourth and fifth starters, respectively, to begin the season.

"We pretty much knew Baker was going to be the middle of April," Sveum said. "We weren't sure how [Garza's] elbow was going to respond, but that was going really well. But now we have this minor setback that could turn into him missing two weeks to a possible month [in the regular season].

Garza missed the final two months of last season with a right elbow strain. The 29-year-old can opt for free-agent status in November.

"I have full faith in the docs," Garza said. "We are on our way back from the stress reaction in my arm last year and everything worked out great there."