Morrow, who hasn't pitched for the Cubs since July 15, threw a simulated game on Saturday as he hoped to be reinstated to the roster. It was one of several times he has attempted to throw in recent weeks.
"Every time he pitched, it got worse," Epstein said.
Morrow could see the writing on the wall with the regular season nearing an end.
"We knew we were running out of time to begin with," Morrow told reporters, "just the nature of the injury and the amount of time it takes to clear out."
Morrow initially went on the disabled list due to biceps tendinitis. Morrow had 22 saves and 1.47 ERA in 35 games before being sidelined.
Morrow said he felt good during Saturday's simulated game but the soreness returned on Sunday morning.
"The next morning is when I knew things were going the wrong way," Morrow said. "I woke up feeling the same way I did a couple months ago."
The development is the second recent blow for the Cubs' bullpen.
Pedro Strop, who replaced Morrow as the closer, is done for the regular season due to a hamstring injury, and his status for the postseason has yet to be determined. Strop has 13 saves and a 2.26 ERA.
--Field Level Media