Thomas Raitt was slashed in the face after he went to the aid of his wife, who had fought to hold on to her handbag during a street robbery.
The couple had been for a drink after shopping in Aberdeen city centre for Christmas presents before setting off to walk home.
Jeeraporn Raitt shouted at Lee Hipson and Albert Massie to stop the attack on her husband and offered to give them money.
Hipson, 19, grabbed Thai-born Mrs Raitt's bag before the pair fled, leaving her husband bleeding and unconscious on the ground.
Mr Raitt, 51, later had 26 stitches in his facial wound, leaving him with a scar.
At the High Court in Edinburgh yesterday, Hipson, who had been freed from a young offenders' institution on licence four days before the knife attack, was detained for six years and nine months.
Lady Clark of Calton also ordered he should be kept under supervision for a further three years for the protection of the public.
The judge told him: "You have expressed remorse and I hope that is genuine."
She jailed Massie, 22, for five years and 10 months and ordered he be supervised for two years.
Both men had pled guilty.