The man was attacked at the Asda store in Killingbeck just before 11pm on Monday night.
He received cuts to the arm during the incident and was taken to hospital for stitches.
A 22-year-old man was arrested at the scene. He became ill as he was being transported to police custody and was taken directly to hospital.
He is currently in a critical but stable condition. The Independent Office for Police Conduct has been informed.
In May last year, a member of staff at the Lidl supermarket in Chapel Allerton was attacked with a knife.