(CBS) The Cubs have placed struggling starter Edwin Jackson on the 15-day disabled list with a right lat strain, they announced Thursday.

In a corresponding move, Chicago has promoted Zach Rosscup from Triple-A Iowa, where he he has a 2.10 ERA and 1.10 WHIP in 30 innings this year out of the bullpen. Rosscup was in the majors shortly earlier this season when he was moved up for a day-night doubleheader. Rosscup made his major league debut last September.

The departure of Jackson from the rotation comes as no surprise, as he’s been the worst starter in baseball this season. Jackson is 6-14 with a 6.09 ERA, and he allowed seven runs in 2 2/3 innings of a loss against the Giants on Wednesday night.

It’s the same injury that affected Jackson late in 2013, and he said he’d been dealing with pain in his past two starts.

“It’s just terrible,” Jackson said after Wednesday’s loss. “No excuse for it. I didn’t help myself, I didn’t help my team.”

In another move, the Cubs called up infielder Logan Watkins, who will help fill in while shortstop Starlin Castro is on the bereavement list after a family tragedy in the Dominican Republic. Watkins made his big league debut in August 2013.

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