Morrow, 35, has been out since last year's All-Star break after signing a two-year, $21 million deal before the 2018 season. His most recent comeback attempt looked promising, but the right-hander never got to the point of a rehab assignment.
"We had that brief respite where we thought it might have a chance, but otherwise, it just didn't seem like he could get over the hump," Cubs manager Joe Maddon said Wednesday afternoon.
Morrow has undergone several procedures on his elbow over the past 13 months, including a recent hydrodissection (nerve decompression), which gave him some hope for a return this season.
"With the way I'm feeling, I expect to progress quickly," Morrow told ESPN via text earlier this month.
But another setback prevented any progress past the stage of throwing at the Cubs' spring facility in Arizona, where he has been rehabbing since February.
"He worked really hard in an attempt to come back and tried a lot of different techniques and procedures and just wasn't able to get over the hump," team president Theo Epstein said.
Morrow has had a career marked by injury, but when healthy, he has been very good. He pitched in all seven games of the 2017 World Series for the Los Angeles Dodgers before signing with the Cubs after the season. In June 2018, he was used by the Cubs in three consecutive games, potentially leading to his lingering issues.
"Maybe we should have had even more conservative guidelines for him or maybe there was nothing we could do," Epstein said. "It's impossible to say.
"He has a significant injury history, which makes it a calculated risk. When you sign someone like that, you know you're going to get quality when he's out there but there's a risk of not getting the quantity."
Morrow saved 22 of 24 games in the first half of 2018, but that's all the Cubs got for their $21 million. They have a team option for $12 million for 2020, but that's very unlikely to be picked up. They signed Craig Kimbrel to a three-year deal in June, and he has taken over as the Cubs' closer.
"We feel bad for him," Epstein said of Morrow. "We feel bad for us that he wasn't able to contribute this year."
Maddon added: "I don't know if he has any plans to do this again. For him and his family, I wish him nothing but the best."