The couple were walking along Portway, in Woodhouse Park, Wythenshawe, when they were approached by a man asking for a cigarette.
They gave the man a cigarette, but when he asked for a light, the 43-year-old husband said he didn't have one.
He was then punched in the head and fell backwards, hitting his head on the pavement - knocking him unconscious.
An ambulance was called and he was taken to Hope Hospital in Salford, where his condition is described as 'stable'.
Det Con Kirsty Cicero said: "This was an unprovoked attack which has left a man badly injured in hospital.
"There may have been witnesses present and I would appeal to them to come forward."
His attacker is in his early 20s, white, with blond hair and a small build. The attack took place around 1am on Saturday and the attacker left the scene along Maisemore Road.
Call police on 0161 8564857 or Crimestoppers, anonymously, on 0800 555111.
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