Jackson, who is in the second year of a four-year, $52 million contract, lasted just 2 2/3 innings in an 8-3 loss to the San Francisco Giants, allowing seven runs to fall to 6-14 on the season. He has a 7.91 ERA in four August starts and admitted on Thursday that he's been battling through the issue for the last few starts. He finally decided it was in the best interest of the team and himself to take a break and make sure he didn't make the injury worse.
"You can try to fight through something as much as you want, especially as a starter," Jackson said. "If you're going out and you're continually going deep in games, then you try to fight through. But I haven't been able to do that. I've pretty much been battling within myself. It hasn't been beneficial for myself or the team."
Jackson said he missed a start toward the end of last season with the same issue. He described the injury as a "soreness" and isn't sure how much time he'd miss, but hopes to be ready to come back in early September.
Jackson said he hasn't been happy with his performance during his time with the Cubs, putting up a 5.47 ERA in 314 1/3 innings since the start of 2013.
"I don't think I've proven to the fans or the organization of Chicago what I can do, what I'm capable of doing," Jackson said. "Maybe a glimpse here and there, but I still think I have a lot of upside, and I still have a lot to bring to the table. I just haven't proved it. At the end of the day, you have to go out and do it on the field."
Jackson has two years and $22 million remaining on his contract after this season.
Castro is tending to a death in his family in the Dominican Republic. He was scratched from the lineup about two hours before Wednesday's game, the first game he has missed this season.
Renteria said he spoke to Castro before he left the team and the young shortstop was very upset about his loss.
Per league rules, Castro will miss at least three days and up to seven days. He is hitting .284 with 13 home runs and 64 RBIs this season.
Sahadev Sharma contributed to this report.