The Pirates have selected Brent Morel's contract and placed Jordy Mercer on the 15-day DL. To make room for Morel on the 40-man roster, the Bucs have moved Corey Hart to the 60-day DL. Mercer has been diagnosed with an MCL sprain and should be out about six weeks, although he won't need surgery.
Morel is, obviously, not a particularly inspired choice to replace Mercer. He doesn't play middle infield and has a long and decorated history of not hitting at all in the big leagues (although he is a good defensive third baseman). With Mercer out, I guess Sean Rodriguez will be the Pirates' backup middle infielder, although he hasn't logged significant time at shortstop since 2012. Something has to give here; surely the Pirates will do something to upgrade their bench in the next two weeks. Some combination of Morel and/or Rodriguez will presumably take over at third for the time being, too, which isn't good. Mercer wasn't having a very good season, but he is, at least, an MLB-caliber infielder, and the Pirates appear to be running pretty thin on those.
The Pirates do, of course, have Alen Hanson, who remains in the minors. Hanson hasn't played shortstop this year, but the Bucs could have promoted him, put him at second, and moved Neil Walker to third until Mercer or Josh Harrison come back. Maybe the Bucs feel Morel's defense justifies leaving Hanson in the minors for now, particularly since promoting Morel wouldn't force Walker to switch positions, but it's very hard to see the upside in the path they chose.
Hart last played for the Pirates in late June and won't be eligible to return for two months after that, so if he ever plays for the Pirates again, it will be around the time of September callups.