A man in his 20s, who was understood to be a fellow worshipper, was arrested on suspicion of grievous bodily harm after the incident on Friday.
Officers were called to the mosque on Clarendon Road in Whalley Range shortly before 1.50pm and the victim was taken to hospital.
Greater Manchester Police said inquiries are ongoing but that the incident is not currently being treated as a hate crime.
Chief Inspector Faz Zaman said: "People will understandably be concerned that an elderly man has been stabbed in a place of worship, but we believe this is an isolated incident.
"Officers will remain in the area for the next few hours speaking to local residents and carrying out enquiries.
"We are in the very early stages of our investigation and I'd encourage anyone who was around Clarendon Road (that) afternoon, or with any information that could help our enquiries, to get in touch."