The 28-year-old Morrow will remain in a no-throw situation until discomfort in the area subsides and he's ready to increase the intensity of his workouts.
"There wasn't anything unusual, there was just some inflammation in his forearm," Blue Jays manager John Gibbons said. "We'll just back him off and rest him until he's ready."
Morrow, who is currently on the 15-day disabled list, was expected to make a return to the Blue Jays' rotation in the near future until he suffered a setback making a rehab start for Class A Dunedin.
The California native threw two innings for Dunedin on Monday and allowed three runs on five hits. He left the game after two innings when he said the pain in his forearm had returned and was immediately shut down.
Morrow had been expected to rejoin the big league club at some point next week but his return date is now uncertain. He will eventually resume bullpen sessions before embarking on another rehab assignment prior to his arrival.
The delay is unfortunate, but a major injury appears to have been avoided and that should bode well for Morrow's second half of the season. He is 2-3 with a 5.63 ERA in 10 starts this year for the Blue Jays.
Gregor Chisholm is a reporter for MLB.com. Read his blog, North of the Border, and follow him on Twitter @gregorMLB. This story was not subject to the approval of Major League Baseball or its clubs.