We don't have info on what happened in Duensing's family to cause this, but I send along my condolences.
Rob Zastryzny has a 3.97 ERA and 1.382 WHIP in 17 appearances at Triple-A Iowa this year, as well as two scoreless innings for the Cubs on May 8 against the Marlins.
Callups from Iowa have done quite well for the Cubs this year and so we hope Rob Z. continues that trend. I'd expect he'll be with the Cubs through this series and Duensing will rejoin the team this weekend in Cincinnati.
I'd also expect the possibility of another bullpen move before this series against the Dodgers ends, as the Cubs have used the Iowa Shuttle to help keep the pen well rested.