The 31-year-old was left critically injured when he was attacked near St Matthews Church in the Moseley area of the city.
The victim initially walked to his nearby home and called police and paramedics but his condition has now severely deteriorated.
It has prompted a police appeal to be launched, with officers keen to find out why the man was attacked and track down the person responsible.
No arrests have been made in relation to the attack, which happened on East Park Way, near the junction of Willenhall Road, at around 11.45am on August 20.
A spokesman for West Midlands Police said: "Sadly his condition became worse and he’s currently in a coma. As you can imagine, his family and friends are very upset.
"At this stage we don’t know why the man was attacked but we’re doing everything we can so that we can find the person responsible.
"We’ve done lots of work already including speaking to people in the area and reviewing CCTV."
Anyone with dash-cam footage, or who saw people running away from the area, is urged to get in touch with the force quoting reference number 20/1609492/21.