Robert Fitzgerald, 24, was also banned from attending regulated football matches in England and Wales for six years after assaulting Stevenage defender Scott Laird.
Last month, Fitzgerald admitted invading the pitch and punching Mr Laird as fans celebrated the club's 3-1 win against Newcastle United in the FA Cup on January 8.
Shock to the system: Scott Laird walks off after the victory over Newcastle
The altercation, which was captured by TV cameras, occurred because the player had previously had a relationship with Fitzgerald's current girlfriend and 'not treated her correctly', Stevenage Magistrates' Court was told. His pregnant girlfriend, Jade Coles, wept as he was led away from the dock.
Sentencing him, magistrates' chairwoman Linda Nadauld said: 'The offence was fuelled by alcohol, and it has become clear from the television evidence that you made a beeline for this particular player.
'We have also heard evidence that a steward noticed disorderly behaviour from you directed towards Mr Laird. The victim was in a vulnerable position as he was in the location to fulfil his duties as a professional football player. The attack was unprovoked and unexpected. The court must impose a deterrent sentence to stop this type of behaviour and conduct on a football field.'
Sentenced: Robert Fitzgerald
Phillip Mansefield, prosecuting, told the court Stevenage fan Fitzgerald was seen 'throwing or spitting' a drink and also throwing a hamburger at the player during the game.
After the match, he ran onto the pitch and was seen to punch Mr Laird 'with such force' he fell to the floor. He said Mr Laird had no recollection of how he returned to the changing room, and was left with a bump on his left jaw after the blow.
In an impact statement read to the court, Mr Laird said: 'I'm concerned that if the male who did this to me is allowed back into the ground that it might happen again. I just want to play football well and enjoy the celebrations when we win, not worry about him getting back on to the pitch.'
Jennifer Browne, defending, said Fitzgerald had acted out of 'misplaced loyalty' to his girlfriend.She said: 'Quite wrongly he took matters into his own hands after an incident between the injured party and his current girlfriend.'
She said the influence of alcohol was a contributing factor in the incident, for which he was 'totally disappointed in himself'. She added: 'Mr Fitzgerald is now in a position where he has to face responsibility. His girlfriend is pregnant. The baby is due in July. He has to find work. He is a qualified personal trainer and now has a job opportunity to work for a major UK gym chain.'
She argued he should not be given a football banning order as the attack arose from a 'personal issue' and could have taken place 'in a nightclub or on the street'.
The court heard Fitzgerald had a previous conviction for grievous bodily harm and was handed a sentence of six and a half years before being released in 2008.
Fitzgerald, of Colts Corner, Stevenage, pleaded guilty to one charge of common assault and one charge of entering the field without lawful authority.
Passing sentence, Mrs Nadauld gave him credit for his guilty plea and said the magistrates had noted his remorse.
He was also fined ?100 for going on to the pitch, and ordered to pay a ?15 victim surcharge.On the football banning order, she said: 'We are making the order because we are satisfied this will help prevent violence or disorder in connection with any regulated football matches.'
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