"I could have been doing too much there," Arrieta said on Friday. "I tend to get a little eager and push the envelope, maybe a little too early."
He told the Cubs about his tightness, so the training staff held him back from throwing. But now he's feeling better and is simply behind schedule. Arrieta and the team want to be cautious.
"I've had a couple of sessions of throwing pain-free," he said. "I feel really good. I think the progress will pick up, but at the same time we do want to be cautious and make sure I am around for the majority of the season."
Arrieta was acquired last season from the Baltimore Orioles and has a chance to become a mainstay in the rotation. He possesses devastating stuff but has been inconsistent with his control.
"The biggest test is going out there and throwing," Arrieta said. "I've done that with no restrictions, no limitations. ... From here on out, it's just a matter of gaining strength."
Arrieta indicated he did not need to undergo an MRI or further tests. Although manager Rick Renteria said Thursday that the Cubs are preparing to open the season without the right-hander, nothing is being ruled out.
"I don't think Opening Day is a far stretch," Arrieta said.
Arrieta was 4-2 with a 3.66 ERA in nine starts with the Cubs in 2013.