The attack took place at the junction of Albion Street and Boar Lane, when the victim was assaulted by a suspect who was part of a group of around 10 other men.
One member of the group was wearing a distinctive giant penis fancy dress outfit and police are now appealing for witnesses who may have seen the group or the attack.
The victim, a 26-year-old man, underwent an operation following the assault after he suffered a shattered shin bone, cracked knee and torn ligaments.
He will be required to undergo further surgery.
Detective Constable Gaynor Burt, of Leeds District CID, said: "The victim was very badly injured in this attack and will still require further surgery. While the nature of costume worn by one of the group might be amusing to some, we are hoping that mentioning it will jog someone's memory and assist in tracing those responsible for what was serious assault that has left the victim with long-term consequences.
"We have been been making extensive enquiries to identify those involved and would like to hear from anyone who witnessed the incident or who has any information that could assist in tracing the group involved, who were possibly a stag party or similar. The costume worn by one of the group is very distinctive and we think anyone who encountered them during the evening would recall it."
The attacker was described as white, about 6ft tall, with a shaved head.
He was wearing a beige or white top.
Another member of the group involved was described as shorter, and of stocky build, with black hair. He was wearing red and white T-shirt.
The incident happened at around 3am on Sunday, April 2.
Anyone with any information that could assist the investigation is asked to contact DC 3146 Burt at Elland Road via 101, quoting crime number 13170151315 or call Crimestoppers anonymously on 0800 555 111.