The victims were drinking in the Royal Oak Hotel on Oldham Road, Failsworth at around 1.15am on October 13 when they were approached by a man not known to them.
The man engaged them in conversation and left, before returning with a number of other men.
This group of men then began throwing punches and using drinking glasses as weapons against the two victims.
One was struck to the right of his face with a glass. He fell to the floor and sustaining serious facial injuries that required surgery.
The other man was struck and suffered a 4cm open cut that required stitches.
The group of offenders then fled from the pub in an unknown direction.
One man is described as white and was wearing a blue/black striped Manchester United top and had short dark cropped hair.
There are no descriptions of the other men available.
Detective Constable James Rothwell from GMP’s Oldham Division said: “This was a violent assault against two men who were simply trying to enjoy a quiet drink in the pub.
“There is no saying that individuals who act in this manner won’t assault someone in future – perhaps with even more grave consequences.
“If you have information, contact police now and help us apprehend these individuals.”
Anyone with information should contact police on 0161 856 8972 quoting reference number 171 of 13/10/18 or the independent charity Crimestoppers, anonymously, on 0800 555 111.