Morrow injured his back taking his pants off early Monday when he got home after the Cubs' road trip.
"It's frustrating any time you can't get out there, especially when you can't go for something as stupid as taking your pants off," Morrow said on Tuesday. "Getting hurt any time is frustrating, but when it's not related to throwing a baseball, it's even more frustrating."
Manager Joe Maddon said Morrow may have been dehydrated after the Cubs' series in St. Louis.
"It's one of those freakish things," Maddon said. "People bend over and hurt their backs all the time."
The Cubs are in the midst of playing 17 games in consecutive days and didn't want to go to Cincinnati this weekend short-handed.
"I don't want [Morrow] to go out there and pitch coming off that right now," Maddon said of Morrow. "There's really no reason for him to rush it back. I prefer him getting 100 percent well and getting back out there when it's right. We cannot go through this weekend, if in fact we play, with less than a full bullpen."
The Cubs are also missing Carl Edwards Jr., who is on the disabled list with right shoulder inflammation. He was making progress, but there's no timetable for his return. Left-hander Brian Duensing is on the bereavement list and could rejoin the team Friday.
Morrow was feeling better before Wednesday's 4-0 win over the Dodgers, but said he'd probably need two to three days to heal, and he understood the decision to place him on the DL. He's also gotten some ribbing because of the way he injured his back.
"Anybody who has had a back spasm knows you can get it doing anything -- brushing your teeth, sneezing, picking up a kid," Morrow said. "I think almost everyone has a story. People hitting me up on Twitter were kind of making fun of me, but also people were saying, 'Hey, I sneezed last week, and I haven't been able to move since.'
"I was just saying it because that's how it happened. I wasn't thinking about it being a joke or anything. It's definitely not the best way you want to do it."
Morrow doesn't feel the pain as much when he's walking around, but sitting is tough. He may be standing during the Cubs' flight to Cincinnati.
So who's the closer?
"It'll be a matchup backside, yes," Maddon said.
Hancock is one of the candidates to close, along with Pedro Strop, Steve Cishek and lefty Justin Wilson. Cishek has been in the closer role for the Marlins and Mariners and is 2-for-4 in save situations this season with the Cubs.
"I don't want to downplay it," Cubs general manager Jed Hoyer said about Morrow's injury. "He had back spasms. He had the same thing in Spring Training. We started treating it the same way we treated it in Spring Training, and I think he was out about a week, 10 days. If things go as we hope, it will be the kind of thing where he'd probably be able to be throwing before the 10 days are up. If he's not going to be ready all weekend, we can backdate it three days, so it makes sense to put him on the DL. He understood that."