Morrow, who has been out since June 13, is in Florida rehabbing from a strained left oblique muscle. The back problem is considered a separate issue, which has been described as minor.
"It has probably pushed him back a couple of days, he's likely to throw his bullpen tomorrow," Blue Jays manager John Farrell said.
"He threw 120 feet today, so hopefully he's on the mound tomorrow. BP, which was tentatively scheduled for Monday, a subsequent change will follow up from there."
Morrow has already thrown two bullpen sessions but is in need of a third before being allowed to face live hitters. From there he will be required to appear in three to five rehab games before being considered for a return to the Major Leagues.
The 27-year-old Morrow is expected to be out until late August. He is 7-4 with a 3.01 ERA in 13 starts this season.