Leicester City supporter Frazer Green threw the flare, which was emitting blue smoke, towards Baggies fans sitting in the Smethwick End at The Hawthorns, striking Ian Topliss.
Mr Topliss needed medical attention after being hit. The flare burned through his jacket after landing in his hood and left him with a blister on his neck.
The game in April was supposed to be a day of celebration for Albion, who had designated the meeting with the Foxes as Jeff Astle Day in honour of the club legend, with Baggies players sporting a special kit that was worn in the 1968 FA Cup final.
But the afternoon ended on a sour note for Albion with Leicester snatching a last minute winner, sparking wild celebrations from more than 3,000 travelling supporters.
Sandwell Magistrates' Court heard that Green threw the flare after Leicester scored before attempting to flee from stewards.
The 20-year-old then grabbed a jacket from a pal in a bid to disguise himself but was eventually arrested.
Unemployed Green was subject to a banning order at the time for running onto the pitch at a game two years earlier, but managed to get into The Hawthorns anyway.
He claimed he made a last-minute decision to go to the match as a friend had a spare ticket. He also said a friend passed him the flare inside the ground.
Ms Sukhi Rai, prosecuting, said: "He was identified on CCTV waving the flare around and jumping up and down before throwing the flare.
"The victim Ian Topliss attended the medical room with burns to his neck."
Ms Rai said by throwing the flare it was inevitable that Green's actions would cause injury.
Other Baggies supporters sitting nearby helped get the flare out of Mr Topliss' hood.
He was left with a burn the size of a 10 pence piece and felt a stinging sensation afterwards, the court heard.
Mr Topliss was at the match with his son, who was 'extremely upset' by the incident.
Green, of Flax Road, Leicester, was sentenced to 20 weeks in prison, suspended for 12 months.
He was ordered to pay Mr Topliss £200 in compensation and must also complete 200 hours of unpaid work.
His football banning order was also extended, meaning he cannot attend games until 2020.
Mr David Hillyer, defending, said: "Foolishness moved onto stupidity and entered high farce.
"He has thrown the thing away not in the direction of the home fans but if you throw it someone is going to get hurt and he accepts that."