Paul Mitchell, of Rubery, Worcestershire, hit Grealish from behind on the pitch about 10 minutes into Sunday's game.
Mitchell, 27, admitted assault and encroachment on to the pitch at Birmingham Magistrates' Court earlier.
He "cannot explain what came over him yesterday morning", his solicitor said.
"His initial foolish intention was to just go on to the pitch and whip up the crowd," said Vaughn Whistance, defending.
Mitchell, of Cock Hill Lane, was also ordered to pay £350 in fines and has been banned from attending any football matches in the UK for 10 years.