Adam Wildman, 21, was 19 when he stabbed one of the boys in the abdomen - penetrating his muscle - and “through” his right arm - causing a severed nerve.
Derby Crown Court heard the injured boy was now unable to straighten his fingers and thumb, while the other teen received a less serious knife wound to his buttock.
Wildman used the knife after seeing his younger friend being “set upon” by four youths also aged around 14 or 15 on a field near the Miners Welfare in Renishaw.
Judge Nirmal Shant QC told Wildman jailing him was a “not an easy decision” however she added: “This a case where you took a knife to the scene.
“What resulted was three individual stab wounds to two different victims - one of them has had a lasting impact.”
Steven Taylor, prosecuting, described how on February 8 last year Wildman turned up carrying the blade after being asked to “bring a knife” by a friend.
Wildman’s pal was expecting a “one-on-one” fight with another youth from Sheffield - who had challenged him to a brawl after a falling out on Snap Chat.
After arriving with the knife and offering it to his friend Wildman was told to “keep it”.
However Mr Taylor said Wildman’s friend was then “set upon” by the group of Sheffield boys and “at one stage he had four of them assaulting him at once”.
Wildman and another friend were then attacked by another boy, receiving “multiple” punches at which point Wildman stabbed the two boys - though neither youths actually saw him holding a knife.
Mr Taylor described how the boy who was stabbed through the arm had been a promising amateur boxer but had now “given that up”.
Charity building caretaker Wildman, who had no previous convictions or police cautions, admitted possession of the knife in a police interview.
Sonal Ahya, defending, said it was “unlikely” her client would have appeared before the courts at all, had he not admitted bringing the knife to the fight.
She said: “The complainant group travelled to Renishaw quite deliberately in order to have a confrontation.
“It’s clear a one-on-one fight was agreed but what the defendant was presented with was (his friend) on the ground being kicked and stamped on.
“The complainant group was far greater in number than was depicted in their statements to police.”
Ms said Wildman’s friend sustained “serious injuries” during the mob attack, including broken bones.
She said: “In panic and fear for himself he made a foolish and dangerous decision to take a knife to the scene.
“He had no intention to use that knife - he intended to give it to (his friend) should he need it.
“The sad thing about this case is this is otherwise a decent young man who had never been in trouble before with supportive parents and a job.”
Wildman, of Carwood Road, Renishaw, admitted possession of a bladed article in a public place and wounding with intent to do grievous bodily harm.
He was jailed for 19 months.